this might only apply to coupes
items needed:
DOW Great Stuff insulating foam window and door
procedure:
remove door panel
spray foam
wait 1hr for cure
reinstall door panel
note: be carefull not to cover any screw holes

ur door will have a firm backing which is lightweight and wont distort or bow the original shape

 

That's the Same Product I use in my attic for insulation..Works good and keeps out the cold weather..