A few years ago, I traded some parts for a fiberglass subwoofer enclosure made by www.BMW2002FAQ.com forum member, Eurotrash (Jason Gibson). It has been sitting in a box for years because I had somewhat mosre pressing issues with the 1600, like keeping it moving. However, I recently bought a subwoofer on impulse, so I dove in.
Buy your sub enclosure from Jason here: http://boltonbliss.bigcartel.com/
Here it is. It's sturdily built and fits under the passenger side of the rear seat. The first thing that caught my eye was that the enclosure is made for a 6.5" sub. Since I impulsively bought an 8" sub, there was going to be some modification.
After removing the installed wiring and fluffy stuff, I traced out the size needed and got to cutting. For those playing at home, make sure you have eye protection and a mask of some type when cutting fiberglass.
Did I mention that my car has seatbelt reels hidden under the back seat? Yeah, well, I'm going to have to make room for those.
Now to lay up some fiberglass.
At this point, many people would sand the enclosure smooth. I, however, remembered that it's going under the back seat and that I have other things to do. A coat of paint will do.
I bought a roll of self-adhering foam to keep the rattles down and placed strips of it on all sides. Done!