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.
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