I am almost finished with a custom fiberglass enclosure that I'm building that tucks into the left trunk wall of a 2003 Honda Accord. It's sealed and holds one 12" subwoofer. I havn't measured the internal volume yet, but I'm guessing it's somewhere around 1 - 1.25 cubic feet. If I have any interest, I'll go ahead and measure it with water to give you an exact number.
It sounds great so far, and it takes up very little usable trunk space (does not interfere with spare tire). However, I'm trying to use my equipment from a previous install and I've decided that I need to use both of the subs instead of just one so I can get more power out of my amp. It would be a shame to just throw away a good custom fiberglass enclosure.
The remaining steps to complete this enclosure (which I will do unless the buyer wants do them) are as follows:
1) Add another layer or two on the baffle to reduce flexing.
2) Sand down some rough edges.
3) Spray a little rubberized undercoat/truckbed liner spray on the inside for dampening.
4) Sand/finish the outside. Let me know if you would like carpet, paint, etc. I originally planned on carpeting it to match my trunk.
Obviously, I don't plan on doing any of these additional steps if there's no interest. So, let me know if there is any interest and I'll add pictures, measure volume, or provide any other information requested.