anyone make their own door panels???
It's called Door Panel Cardboard available at automotive upholstery / fabric supply shops.
Black waterproof treated cardboard that you can sew with an upholstery sewing machine. Takes contact adhesive / fabric adhesives well. Can even roll your fabric over the edge and staple it with the short 3/16" staples at an angle.
I know Pyramid Trim Products in St. Paul, MN has it.