FYI, my enclosure fits 2 x 10" JL Audio subs, it's 8 layers of fiberglass molded to the spare tire well. The MDF baffle fits flush with the trunk floor. The trim piece that finishes it off raises the whole thing another 1" to include grilles that protect the subs. I have spacers on either side to level the floor off and the stock carpet fits over the whole thing. It weighs 15 lbs more than the spare / jack combo which is not bad at all...
Here are various stages of the buildup, it's not upholstered yet... But it sounds pretty good for something that you can't see!
Dynamat inside the box

Mounting the baffle and filling in the gap

The assembly in the trunk, fits flush with the lowest points of the floor

Filling in screw holes + initial testing (note the polyfill in the box)

Weather stripping to prevent air leaks

Drivers mounted, surfaces smoothed out, awaiting upholstery

The $$ issue:
- Cheap subs 200$
- Leftover MDF at Home Depot 5$
- Fiberglass supplies 50$
- Countless hours... did everything myself