I first mocked up the box in cardboard, then duplicated it in aluminum.  At least the shape fit both sides equally well when it was mirrored.

My intention was to add flanges all the way around that would be attached to the door panel .  I have some Velcro with very sticky backing, so I'd use male on the door panel and and female on the flanges.  After a little fiddling, I decided I only needed the Velcro on the top and bottom, which made my work even easier.  Too bad I didn't think to put the bottom flange on the box to begin with.
Before the box was fastened permanantly, I lined the inside with some short-pile carpet.

After I "stuck" the box onto the backside of the door panel, I glued some foam onto some strategic areas.  This not only helps keep the box in place, it "de-rattles" the system and acts as a sound-deadener.

Then I reinstalled the door panel.

Back padding


Replacing the door panel and final hole trimming