I ran across this and wanted to share for those who've been looking for something affordable. It looks like it could fit under the the 3rd row seat with little or no modification.

http://www.titantalk.com/forums/titan-visual-audio-discussion/65346-great-sounding-sub-enclosure-takes-up-very-little-space-pics.html#

Looks good but you get no cargo space mine as well go big...

Looks good but you get no cargo space mine as well go big...
It could also be put in the cargo area or wired to pull out and set on top when the seat is folded down.