Just messing around to see what could fit and built a skeleton for a box under the rear seat. Downfiring is about what I expected with 4ish inches of clearance. But I was surprised when I measured the front, I made it with a slight angle up front which keeps with the angle of the seat. It looks like you can use any 8" sub you would like without any seat lift. Frame clears everything under the seat including the handles. Will post some measurements later. Pic is of a DC 8" subwoofer.
Looks like it would work just fine.
No where near enough clearance with 1/2" or 3/4" plus clearance for baffle.