I built a large box a couple years back that held two 12" subs, amps on the side, tools and air compressor/tank. I sacrificed a ton of cargo space and was carrying around an extra 100lbs (just the weight of the material). I'd also like to have the option to toss the air mattress in the back on camping trips.
When I took the big box out, the rear end raised 2 inches, cheapest lift ever! I wanted to build something as low profile as possible. Hard to do for a 12" sub. The only place suitable was the passenger rear corner, driver's side has the fill neck.
Basic dimensions of my new box are 15x15x6.5. The 45deg cut is 10" in. The sub is 5.5" deep.
Materials used were leftover 1/2" plywood from old box and 1x2 frame pieces that were actually in a bundle of kindling that I had.
The box will be counter sunk into the wall about 6".