Spare tire (duh)

Aluminum foil

Tape (scotch tape or painter's tape works well)

Scissors

Fiberglass mat

Latex gloves

Fiberglass resin and hardener (this is in the paint section of Home Depot - I went for the cheap stuff but I'm sure there's more high quality resin elsewhere)

Disposable paint mixing cups and stirrers

Cheap disposable paint brushes

A 2'x2' square of 1/2" MDF (medium density fiberboard, you can find this in the lumber section of Home Depot)

Basic powertools: jigsaw, drill, angle grinder or some sort of sander, dremel might help

PAM (that stuff you spray on cooking surfaces so that the food doesn't stick)

Underliner spray (I found this at AutoZone, ranges $4-$8 per can)

Bondo putty (optional, see below)

Sub speaker (I'm using the Pioneer TS-SW2541D) - you'll need to make sure it's low profile and doesn't need much volume in the enclosure

Terminal - make sure it's sunken, such as this one

A grill to protect your sub (optional). I'm using this one, which tucks right into the rubber surrounding the sub; no hardware required!

A clean place to work; I spread out a big canvas on my garage floor

Mineral spirits in case you get resin on your skin, lots of paper towels

Soldering iron, solder

Inspired by this thread (thanks niko3257!), I decided to try to make my own in-spare sub box. I've never worked with fiberglass before this project, so I thought this DIY might be good for other newbies.Supplies:That should be enough to get you started; you'll obviously need to get an amp (I'm using the Sony XM-1S ) and the appropriate wiring (I picked up this kit ).So here are all the supplies I needed to start the fiberglass process:Use the aluminum foil and tape to make a circular nub where the bolt that holds the spare can sit. You don't want to make this taller than necessary, since your sub will need 3" mounting depth.Cover the rest of the spare with aluminum foil/tape and spray a light layer of PAM over the whole thing. This will make removing the foil a lot easier later in the process.Cut the fiberglass mat into 3"ish strips and follow the directions on the resin can to do your first layer of fiberglass. I found that mixing 10oz of resin with 5.5mL hardener was about the perfect amount to work with - I'd use it all up right before it started hardening. Basically, just lay the strips of mat in the tire and use a paintbrush to soak the strips with resin. Do one layer and let it dry according to the directions.Once the first layer has dried, put a light coat of resin on that, then lay down your next layer of strips. Soak with resin as before. I experimented a little here and went with a heavier mat, which was much more difficult to work with. I'd stick with the lighter mat for all three layers if I were doing it again, but the heavy mat does build up the thickness quickly.Let it dry. At this point, the structure should be strong enough that you can pull it out, put the spare away, and remove the foil/tape.If you want some instant gratification for all the hard work, you can see what it looks like in the spare.I did one more (third) layer of the light mat.Follow the directions on the underliner spray to give the interior a nice coat. This part probably isn't necessary, but I like to think it'll help seal up the box.Use a jigsaw to cut out your MDF. For my GE (not sure if GD is different) I made the outside circle an 18" diameter, I think. My sub came with a template on the box, so I cut that out (don't throw it away, you'll need it later). The terminal should be easy to trace after that. Drill pilot holes for the screws so that you don't risk cracking the MDF later.Spray one side of the MDF with underliner spray.With the underliner side down, put the MDF back in the box and resin the flaps of fiberglass mat over the top. I got a little sloppy here because some of the resin had dried on the flaps, which made them hard to shape. If you're more careful, you may be able to skip the next few steps.Here's where some of my flaps goofed up because they already had some dried resin holding them in a strange shape. If you can avoid this, it will make your job a little easier.I used a combination of dremel and angle grinder to cut/sand down the parts that were sticking out like that. There were a few places where the resin hadn't totally sealed the space between the MDF and fiberglass box, so I used Bondo putty to fill in those gaps. If you're more skilled, you probably won't have to do this part.After shaving down some of the resin drips, it still fits great in the spare. The cargo floor won't lay totally flat anymore, but you can hardly tell. I'm putting the speaker off to the right since the spare isn't totally centered in the trunk.Spray the exterior with underliner spray, following the directions on the can.Now to cut the hole in the cargo tagboard. Put the box in the position you want, then cut a rough hole in the middle (large enough to get my hand through with room to spare). Use a graphite pencil to trace the size of the hole on the underside of the tagboard.Matche up the inside circle of the template to the circle you just traced. Then trace around the outside of the template. This is where you'll want to cut, since the entire top of the sub will need to come through. You'll also want to rip off those foam rubber feet (the flat ones, not the cube ones) since the tagboard will now be supported by the sub.Cut out the hole (I used a jigsaw) and put the tagboard back in the trunk.Now you get to put your sub together! You'll need speaker wire to connect the terminal (soldered) to the sub (clipped). Screw everything together using the pilot holes you drilled earlier. In these pictures, you can see the grill I used. I just tucked it under the rubber piece on the sub, which required no hardware (although I had to cut away some of the rubber to make it sit flat).Run your wire, install sub! I'm planning to do a writeup on my complete audio setup, and I'll link to it here when that's done.Here's what my finished product looks like.I'm hoping to finish head unit install tomorrow, then I'll be able to talk about how it SOUNDS. This DIY will be edited/continued with that info. Questions/comments welcome!