Lets see some pictures mangJL Audio CP108LG-W3v3 FlatWedge™ slot-ported enclosure with one 8" W3v3 subwoofer at Crutchfield Signature
The JL Audio CP108LG-W3v3 is a shallow mount 8" subwoofer in a JL Audio designed ported box that is only 5 1/8 inches deep without the speaker cover. With the cover it is 6 1/2 inches deep, but can still fit under the rear seat, towards the front of the seat.
I just installed mine today with a 210 watt amp and it sounds pretty good. Its not going to rattle windows, but with some hip hop it created enough bass to vibrate my rear view mirror.
The box is small enough to go under the seat and not leave room for other things to go there. Its light enough that you could take it out of the truck if you anticipate needing the extra storage space too.
Just wanted to share another option with you guys. I'll take pictures later on for you guys, I want to put the seat back in so I can show how much space there is.