here's what I did:
1. Box is 20.5 x 12 x 8, used 3/4" plywood
2. Glued all edges and used 2" wood screws
3. Sanded the hell out it
4. Lined the inside with dynamat equivalent and sound deadener
5. Drilled small hole on the side for the speaker wire exit (sealed later with hot glue)
6. 3 coats black semi-gloss acrylic paint
7. Cut hole in fibreglass after removing carpet from inner wall
8. Placed box into compartment. You will notice if you look there is a little lip... Box rests on that.
9. Holding box in place, trace cut out through fibreglass hole onto box
10. Take box out, cut hole in box
11. Cover face of box (except hole, of course) with dynamat.
12. Make copy of dynamat, attach backwards onto face of box using 3M spray glue to attach. This is my "gasket" to the bare fibreglass.
13. Put box back in, line up cut outs
14. Place driver in hole, line up, predrill holes, double check alignment, screw in. Note I did not use supplied screws as they were not long enough to get through fibreglass and wood.
15. Cut extra carpet from compartment away, reattach remainder.
so that's it... Any questions, let me know