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· Mod. --- I'm watching you
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Discussion Starter · #1 ·
Since I'm new here, I'm just going to state that there are many ways to make a sub box, this is how I made mine. If you disagree with the way I build it that fine but please don't ruin the thread with a flame war...Thanks John

Already... since I've joined the board here a few people have asked me about building a fiberglass sub box for the rear of the Discovery. So I figured I would do a write up on how to build one. This is not exact but fairly close.

Now since I haven't started the build of the rover's system yet, I'm using my S2000's sub box build. Its the same thing either way just your shape will be different. Once I finish the rovers sub box I'll change this thread to show that one.

So here's how to build your own sub box out of fiberglass from start to paint or vinyl.

So where do ya start... well first thing your going to need is a list of materials and tools you'll need, its kind of a longer list but a complete one:

1. Stitched weave fiberglass or at least tight weave - (stitched weave is much better than the crap at HD or Loews)
2. Mold fabric 7 oz MICRO
3. Fiberglass 4mm Filler mat
4. Polyester grade fiberglass risen
5. Mold release wax <- Very important!!
6. 1/2 inch MDF (Medium Density Fiberboard)
7. Black OEM hot glue
8. Wood screws 1"
9. Rage Gold bondo or Lightweight bondo (possibly Tigerhair-bondo with fiberglass strands in it for strengh)
10. Acetone (to remove the fiberglass off the roller)
11. 2 part glue (something very fast acting les than 20sec set time, I recommend Select Products Speed Glue and activator 10 sec)
12. Box of rubber/plastic gloves!! (or you'll be pealing the skin of your hand for about a month)
13. High build primer (gray)(high build primer will fill in all the little pin holes you get in the bondo)
14. Paint or Vinyl

1. 10-15 cheapo 2" Wal-Mart brushes... roughly .29ea. type of thing.
2. Painters tape 2' wide (the blue stuff because it’s not that sticky but seals up to itself pretty good)
3. Cheapo mixing cups (Quart size for the fiberglass risen mixing)
4. Sandpaper from 40 grit up to 800 grit
5. Glue gun (small but must hold 3/8" OEM glue sticks)
6. A router table is a good idea but you can do it without it. Just buy the ring for the sub.
7. Fiskers to cut the fiberglass... this stuff will dull out regular scissors in like 5 minutes... get the Fiskers.
8. Bondo spreaders 3 pack
9. Sharpie's... never enough ;)
10. Painters mixing sticks... for mixing the risen.
11. Fiberglass roller.(this helps remove bubbles from the fiberglass mat once you brush the resin onto the mat)
12. Dremel/die grinder with cut off wheel. You'll need it trust me, as well as a lot of thick cut-off wheels
13. Breathing mask with replaceable filters (resin will give you one hell of a headache and fiberglass will cut up your insides)
14. Small air nail gun would be good but you can get away without it.
15. Bondo shaver
16. Power/Air hand sander is very helpful but not needed as you can do it by hand but it takes forever, and ever and ever and ever ;)

Ok Let's get started:

Before you do this in your car you should do a small practice run in an old bowl or something similar, half a milk bottle or something. Follow the directions below but in something else before you start on your car. Doing this will give you a feel of what’s involved with working with fiberglass.

While working with fiberglass you should try to keep the workshop/area around 60'+ for the resin to dry properly so it’s not sticky, 70' or higher is best.

1. You can use a few different things to make a mold of the trunk well. My personal favorite is painters tape right on the carpet. (You can also use tinfoil sealed with tape, both will work) I used both together in these pictures just to be safe) You should overlap the tape by half. So lay the first piece of tape and the next piece should cover it with the next piece laying piece halfway over the first and so on until the entire well including the top lip and about 6inches all the way around is totally covered. Then do the exact same thing but going parallel to the first set. Once you have 2 full over lapping sets of tape. Next thing you MUST do is put 2-3 coats of mold release wax. Put it on nice and heavy.

2. Next thing I want you to do is cut a nice size section of the stitched fiberglass mat so that you can lay inside the tool well. Take a large section of fiberglass and place it inside the well and cut it so it sits flat and smooth inside the well like this:

Ok once you have the mat cut to fit without it being bunched up (it should be fairly smooth looking) in the well, you’re now ready to mix your first batch of fiberglass resin.


3. Mix a small batch of risen and hardener, about half a quart or so. Warning if you mix too much hardener into the risen at once it is possible for it to catch fire and or melt the cup... so be careful the stuff can get very hot. Follow the direction of the manufacturer.

Once mixed, brush the resin onto the mat. Starting at the top edge, because it’s going to run down the sides and it will want to pool at the bottom. So continue to do this mixing more resin until the mat is completely soaked all the way through. Work fast pal... Once you've done this your going to have to let it setup nice and hard.

4. While the risen is hardening up you can go ahead and make the ring for the sub if you didn't buy one to fit. (my brother made this one on the router table but he's really good... that's why he does it he's awesome on the router table)

5. Next you can also start to either draw or at least envision what you have in mind for the box. In this case we decided to make the box ported this time and to also add a "cap holder" into the box...

6. Once the box bottom is dry you need to stiffen it up to make it strong. The best way is to use a 4mm fiberglass filler mat that will suck up a large amount of resin and make it very strong and thick. Now with this mat you really should have an extra set of hands because your going to need someone to mix the resin while you spread it onto the mat. Now you'll need to cut the mat into strips BEFORE you start mixing the resin. You should cut them into strips as long as you can to make it stiffer. Fill in the spaces with smaller sections but you really should fill it in as best as you can. You can soak them in the resin and then lay the strips into the shell of the box, keeping them as close as possible so in the end it will look as if you laid another layer of thick layer of fiberglass down.
Sorry I don't have a picture of this... I deleted it by mistake:banghead:

7. Once that is dry, lay another section of stitched weave fiberglass mat over the filler mat just like you did in the section 2. This will complete the bottom section of the box.

Let it fully dry between each section; overnight is usually good; remember 60'+ temp

8. Ok so at this point the box bottom should be fully dry and very stiff. Its now time to remove it from the car. This is how you do it, grab the back of the box closest to the gas tank/spare tire well and pull up towards the rear bumper. You kinda have to rotate it out because the inside section under the trunk latch is indented so you can't just pull it straight up and out, so you must pull from the back of the box. Once you pull it out... and it will be kind of a pain, it should look something like this (the inside of my box is painted black)

Do not trim the top just yet. I know you want to but don't do it just yet. You need to decide what your box is going to look like in the end. However this should be done already. You need to make 2 MDF rings to go on the inside of the base. This will do a few things. 1 keep the shape of the box accurately shaped to the well and 2 it helps to keep the box stiff and adds strength and gives you something to shape to.
Here are a few pictures of the process:

As you can see this box is a little complex but you can see the process. But you need to be able to brace the sub ring up in whatever shape/style you'd like. Ok once you have the box ring the right size you can now cut the extra fiberglass off around the top of the box at whatever height your comfortable with. You do this using a Dremel or a die grinder. Once you've cut the extra off the box your left with just the base which has been cut to fit, with a dual MDF ring that fits inside the fiberglass base, the fit should be snug. If it's a little to big don't cut it down to size, sand it down to the right size it's safer that way. Once you have the right size screw them together using woodscrews dipped in hot glue. (don't forget to drill the holes for the screws so as not to crack the ring), then secure it using the OEM hot glue Glue the ring in place with the second ring above the edge of the base. Once you've glued it in place, place the box back into the trunk well to make sure you’re happy with the it and double check to make sure it still fits, and that its level and stuff like that. Once that's done remove it again, mix up a batch of resin again and resin the ring in place mostly on the underside of the ring. Let it dry.

9. Alright by this point you've made the base, made the MDF rings to stiffen it up, glued them in and resined them in place, you've checked the height of the box in the trunk well and your happy with it. Next you’re going to make a few 'supports' for the sub ring to sit on. you can see the ones in the pictures above, so you get the idea. They should be glued in place and once you put the sub ring on it should be fairly stiff. You should not be able to wiggle it back and forth. Reason is if you can when you go to wrap it might break. You'll see what I mean soon enough.

10. Alrighty. Now its time to wrap the box with the Mold Fabric. This is very important because if you rush this or mess it up you'll be in bondo hell for days trying to fix it(this is one of those things do it right once the first time...) Ok get out the speed glue and activator and a section of mold fabric roughly 1 foot larger around than your box,... the extra will be cut off, but when you’re trying to stretch the fabric you'll be happy you have the extra to hold onto trust me. So starting in the middle of the box in the sub ring(you always start in the middle) Place a small amount of the speed glue on one section about 4-6inches long on the sub ring itself. Once this is done spray the activator onto the mold fabric and stretch it and stick the glue and the sprayed section together. It will form up in less than 10sec of you have the same stuff I have. Once it does, continue to glue down the mold fabric around the inside of the sub ring. Once you've glued it down, it should be like a drum... or at leased fairly snug. If you you've added a port do the same thing. Pull and stretch the fabric away from the sub ring. so it molds nicely. If you’re trying to make valleys like I did, just stretch glue the fabric in place. Make sure you pull the fabric nice and tight so it doesn't have any 'ribs' or high points in it... because if it does you have to build it up with bondo. Once you glued the interior points up you can now stretch the fabric down and onto the sides. Start on whatever side has the shortest distance from the ring to the edge of the box. Pull it tight, and glue the fabric onto the side of the box using the speed glue and activator again. Do not use hot glue it leaves high spots and doesn't stick nearly as well as the speed glue.
Moving from the shortest to the longest section stretching the fabric onto the side of the box glue the entire top onto the base. Once you've done this you should pretty much know what the box will look like in the end.

11. Now we go back to section #3. Start mixing again. When you’re applying the resin to the mold fabric you need to make sure you get it to soak all the way though to the underside of the fabric. (DO NOT brush resin onto the sub hole... because you'll just be making a lot more work for yourself when it comes time to remove it.) Brush it on real thick. It would be best to have a helper either mixing it for you or brushing it on for you, because you’re going to go though a ton of it. Once it dries do another layer. Once that dries cut the sub hole open and remove the fabric. Next with your hand (no glove) reach into the box and see if you got it to soak though if you missed a few spots mix up another batch and brush it onto the inside of the box. Once this dries fully it should be as hard as a rock.

12. Then once its fully dry break out your favorite CD, sit down and start sanding with the 60 grit to cut down the high spots. You'll know what I mean when you get this far... once you get it pretty well cut down do another test fit.

Very good! Now comes the fun part,... using the bondo. Rage Gold has been the best stuff I've ever used. It’s so much easier to sand than regular bondo trust me on this one. You can sand it by hand, unlike the regular bondo. Go over the box filling in the low spots and smoothing it off. If you have gaps from the box to the trunk well tape it off again and fill in those few spots so when its done you'll have a nice snug fit, and it will look sweet!.

Once you get the shape right and the holes all filled, start sanding... start with 100grit and move all the way up to 800-1000 grit. It should almost feel like clean paint if you've done a good job.

13. Now you have to wire it up. Drill a hole just big enough to fit the wire through (12 gauge is good). Once you have about 2' in the box and a bunch outside the box go ahead and seal that bad boy up with some fiberglass resin and bondo.

14. Once that's done you can either prime and paint it or vinyl wrap it. If your going to use vinyl make sure you get good stuff. A brand that will stretch a little, to help form it. (If you use vinyl you'll need to sand the edges down a little extra to make room for the vinyl around the outer edge of the box.)

However for this one we painted it. I did this with spray can paint(with a little spray can handle thing I got at Loews) because I was too lazy to build a mock up spray booth in the garage to use the paint gun. But I primed it about 4 times with the high build primer, sanding in between each coat. It should almost look like you painted it before you do paint it. But then you paint it and clear coat it an.....d There you go!

15. The last thing to do is install the sub and you’re ready to install it.

I hope this helps some of you under the process. As I stated earlier I recommend Select Product for material. I understand they can be a bit on the pricey side but they have good stuff and so far it’s always in stock.

You can buy the below listed items at


Good luck with your projects!! I will update this when I start and sub box build for my disco.

If have anyone has any questions just let me know...

· Registered
1,744 Posts
Get stuff! As I'm a mad customiser (who already has sub enclosures), I love your work. Your detailed post allows me to look at fabricating other interior projects. :clap:

· Mod. --- I'm watching you
319 Posts
Discussion Starter · #4 ·
Disco said:
Get stuff! As I'm a mad customiser (who already has sub enclosures), I love your work. Your detailed post allows me to look at fabricating other interior projects. :clap:
I can do a post on how to make these lower door pods if you'd like. I made these out of fiberglass as well.

· Wheeler
1,837 Posts
darkknight1999 said:
If you disagree with the way I build it that fine but please don't ruin the thread with a flame war...Thanks John
Never. No flaming on LRO. Thanks for taking the time to post such a great thread ! :buttrock: :drink1:

· Mod. --- I'm watching you
319 Posts
Discussion Starter · #6 ·
Thank god! :clap: :drink1:
I'm a moderator on and you'd never believe the morons that show up over there... :confused Sometimes they just make you crazy, but I've been there so long now that I've learned how to deal with them.

· Mod. --- I'm watching you
319 Posts
Discussion Starter · #8 ·
I'll do one of those as well. It takes me a while to do the write up, but I'll get it done for ya.

· Mod. --- I'm watching you
319 Posts
Discussion Starter · #13 ·
Thanks again guys! I've build some nice new door panels I'll have to post up sometime. Its been a while since I've been on here... far to long ;)
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