I think if your are going to go the way of a box in the trunk you might as well put a 10" sub in it. I have one ten in my trunk and it sounds pretty good with one of the back seats down, but when I put the seat up it muffles the bass quite a bit, and I think with an 8" and the seats up all the time, you might be a bit disapointed and wished you had gone with a 10". plus, a 10" isn't really going to take up any more space than an 8' because you can get something that will work in a .66 cu. ft. box which is very small. Thats the size I have in my trunk, however it is placed behind my amp rack. Here's a link to show you, I still have plenty of trunk space.
http://www.pbase.com/image/3548119
