$18 for a 4x8, 3/4 on an inch sheet of MDF at Home Depot, you can also have them cut it for U, but I wouldn't recommend that. The
measurements that I gave them were specifically for my trunk, and after
they were done with it, I had to throw it away. Thay also sell the carpeting,
spray adhesive (Only use 3M), screws, and just about anything else you
need to build a box, except for woofer screws, but if your speakers are new, they should already have a set included with them. It only cost me a little over $40 to build my box, but at a an audio shop, I would've been charged anywhere from $150 to $400 just for a regular rectangular box.
Mark-up prices ara BITCH!!!