that's a picture of the box that i used to have. but, i'm thinking that the best way to go is custom (mine was i think 200 or 250 to have built, but they did a great job. also, some people just set the box in their back seats, or just remove the backseats all together and put a big box (like you're talking about) back there.
shallow mounting subs will also help. check out jl w1's. i know those are very shallow mounting subs and should give you lots of room. and, go ahead and check out the car audio of the boards too.
good luck
_isiah