Here's what the boxy 15gal plastic (POS) version looks like in the back of my Jeep. (sorry for the webshots and bad quality pic but it should give you a general idea)
http://community.webshots.com/photo/...4081VLKbDGhVZc
This pic will give you a pretty good indication of how much room using a cell frees up the rear end.
http://community.webshots.com/photo/...0302BjuUdnzQdf
Jeeper