Cowlboard Waterproof Cardboard Door Panel Board(s) [32" x 48"]Here is a waterproof panel board.
https://www.automotiveinteriors.com/ProductDetails.asp?ProductCode=cowl-dp
I used 1/8" ABS panels from a plastics supply house. Textured on one side and smooth on the other. Easy to cut, drill, and staple.
I literally just went out and bought the wood yesterday. I'm using tempered hardboard (Lowes) that I'm going to treat with Thompson water seal. The hardboard seems to be the closest match to the original ones that were falling apart in my '40 Deluxe Sedan.
Now I need to figure out how to slightly shape the hardboard for some of the areas.
Water and steam for shaping and forming. So do all the fitting before applying the water proofing. Also Thompson is paraffin wax and will make it difficult to get anything to stick to it.
Good point on the water sealer. If it were me, I'd either just apply several good coats of paint after shaping or apply laquer, poly is not water proof, or marine spar varnish and then apply upholstery fabric and trim.
Pros and cons. The cardboard ,wood and yes the masonite are easy to work with and are some what sound deadining. They are good at holding staples and glues and tend not to viberate and rattle. The down side is they dont like any moistier. Over time even humidity will cause warping to some degree even in waterproof masonite as the surface is water proof but not where you cut or drill it. The plastics don't have this problem but they are a little harder to work with as they don't glue staple or hold tacks as well. I had a customer complain that he couldn't stop the hard plastic door card from making an anoying vibration rattel on hot days until coverd the back side with thin foam. Just some thoughts.