Why not just do a half under seat rear sub box? Why not use a JL stealth box? They sound OK for what they are.
My buddy had a 00 3500 dually that we went wild with in terms of his stereo. We started out under the back seat, then we took out the back seat, and we ended up building a middle seat box. (the truck had 2-15's and 3 12's when he sold it) The middle seat box worked out neat, but kind of made the presense of the bass feel a little "off balance" I guess. I dont know how to describe it. It sorta sounded you had sound on your right side (from the pass seat) and then a different sound on the left. Kinda weird, not full and even.
Oh yeah, what DM said about sealed boxes. They are better for all types of music. If you want to hear one tone, go with a bandpass or ported enclosure. If you like clean, clear and precise music, sealed is the way to go IMO.
-Jeff