Well, you need the resin and hardener and some fiberglass cloth or mat (or both). Then you need a bunch of cheap disposable brushes to apply the resin, some latex gloves (not completely necessary, but a good idea), some fleece (t shirt material) if you want, as you can use it to build up a thick layer quickly, and a mixing bowl that you don't want for any other use. Also, a lot of masking tape or duct tape, and some kind of mold release. Pam cooking spray actually works well for this.
You can get the resin and hardener as well as the cloth at Home Depot, Lowe's, or most auto stores like Autozone. You can probably get it at a marine supply store as well. If you have a Harbor Freight around you, they sell boxes of brushes really cheap.
If you're doing a sub box, you'll also need wood to make mounting rings and probably some wooden dowels to help get everything positioned the way you want it.
Keep in mind that fiberglass is strongest in curves, so if you have a big flat section, it'd be better to use wood for that part. It's not the hardest stuff to use, but it can be difficult to use well, ya know?
If you decide to do it, make sure you're in a well ventilated area, as the fumes aren't the best for your health (you can use a respirator if you want or need to). Your car will probably stink for a few days after you make the initial mold in the car. Then when you get to the point of finishing it, get ready for seemingly endless sanding...