The 911 is a tough nut to crack when it comes to subwoofers. The most common placement does seem to be the rear seat. The problem is you lose the use of the back seat. Of course that seat isn't THAT usable in the first place, unless you are about 4'2. Here is an example of a rear side panel.
Here is a pretty nice example of a rear seat replacement box.
There is a worklog on the build here..
http://fiberglassforums.com/build-pi...1sc-build.html
I have been a member of the fiberglass forums for a while, it has a ton of information, tutorials, and examples of this kind of fiberglass work.
What ever you decide to do .. I want pics!