When not using the car at the track, I have one of these that I cut in half, so I'm only using one 12" sub...
Rockford Fosgate P2L-212 Punch Stage 2 dual 12" ported enclosed subwoofer at Crutchfield.com
The bass is RIDICULOUS and reasonably tight... too much for me to be honest
The back of my car is completely striped in the back for track purposes, so it sits on the same level as where the spare tire sits... it has about 2-3 inches of clearance above it, below the hatch... would not fit if i hadn't striped the back