[Tempest67]

Here is what I did,

start with welding a 'tack strip' to the rear door frame. (used to attach the speaker panel)
then bend a piece of 18 gauge to form the rear corner and attach to 'tack strip' and the rear support strap.
Cut the 6x9 hole, and make a 'lid' for the enclosure,
then insulate/carpet the entire piece up to the 'chair rail' as the carpet will 'bend' easily to fill the corner.
do that to both sides, and make a rear panel (1/16 mdf) that butts up to each speaker enclosure
I also added 2, 4 inch round speakers to the dash.

Hope this gives you some ideas