Well I started by using a kind of papier mache, newspaper with woodglue.
Now I would go straight to MDF with a card template.
Attaching back to sides I used selftapping screws then glassed over the joint. But now I would make the back in MDF then attach the sides with screws and any odd shapes and corners glass over.
Its a solid as a rock now!
The sound is great! The 10" is as good as the 12" i've got in my other car!