Your box looks sweet.
I went with the Kicker Wakeboard boat subwoofer. It is a solid Poly box, UV resistant, waterproof, and fit in the spare area perfect. I cut a hole in the extra rear door cover that I had from the abandoned VX that was at the local Isuzu dealership. It works like a champ, and I mounted a boat cleat to the box for easy portability.
Got it for 50 bucks at Circuit City, it was a display, never even been hooked up...

http://www.crutchfield.com/p_20606SK...=Kicker&tp=972