I need to create an irregular-shaped speaker enclosure to go in my car, a sub-woofer box. Most car audio guys just tape up the area, wax it up, and start to lay the chopped stranded mat into place.. sometimes strips of normal glass and layer it up.. usually these end up about 10mm thick..

Just be aware that if you are planning on using polyester resin, you'll either want to be sure that your cars boot was painted with 2k paint, or you'll want to use PVA release agent in addition to the wax. Wax on its own won't stop the styrene in the polyester from attacking the solvents in 1k paints.