I was seriously considering the
Select Increments Opti-Pods for awhile, but they seem like they'd take up too much space for little benefit. You can't get a subwoofer bigger than 5-1/4" in the Tuffy boxes, and 6-1/2" in the Opti-Pods. I wonder if you could fit 8" subs by cutting holes in the sides. That would take up all of your storage space, would require sealing the grille in the front, and would open up the possibility of rear-seat passengers damaging the cones. But... it's an idea.
I just went with a 10" slim sub under the seat. I needed to cut out two seat supports, but it works really well.