"Masonite" is the composition board. After a church remodel, we had some scraps of a thin (maybe 1/16") fiberglass wall covering that had been installed in the bathrooms. It will do what you want. However, neither of these materials would be stitchable. I think an uphostery pro could insall with glue. Just a thought.
'92 Limited 34ft (now sold); '96 Dodge Cummins 4X2, 5speed