Hmmm. I had a Chevy Spectrum with the passenger seat removed for a while. LOTS of space in there.
A friend of mine has a gen1 Metro with the back seat and passenger seat removed - the thing is as spacious as a pickup truck!
Actually, when it comes to Eco-Car-Camping, I think it's hard to beat this.
Plain cloth tent. One end of rope goes to a tree, other end goes THROUGH the open windows of the car and ties to the roof itself.
The car also had a "powered by Ethanol!" sticker on it.
Come to think of it, this is the type of forum where the owner is probably right here somewhere.....