dude, it may be a little car, but the trunk in it is MASSIVE. I have 48" wide, 22" deep, and 19" tall (11.6 cuft) to work with without making an odd shaped box.
EDIT: If you take into account wood thickness, make that around 9-10cuft. I'm also realizing that an 18 wont fit the way I want, so i'm thinking of 2 15s instead. luckily for me the air you need for 1 18 is the same for 2 15s