You can do a lot of cool stuff with fiberglass. It all kind of depends on where you want the sub and if you want your spare tire still. http://www.tccustomz.com/inc/sdetail/94
This is a pretty cool looking fiberglass enclosure that wouldn't be to difficulte to duplicate.
Either side of the hatch/trunk makes a good spot to build a glas' enclosure. The irregular shape of these corners offers a lot of air volume for a sub when you can sculpt the box to fit the car. If you need some tips, just e-mail me. Getting started doing this just takes some nerve and patience. Most any competent body shop can handle the finishing details for you.
I have attached a picture of the beginnings of making a mold of one of my hatch trim panels. I will be glad to explain the process.