I don't know exactly what you are using Bob. You say it is not masonite but is a masonite type material. Obviously it is not a product the OP can go to the store and ask for. Years ago I used 1/8 tempered masonite that is more water resistant than regular masonite but it is hard to find, expensive and still will deteriorate eventually.
The PVC board is 100% waterproof, formable, with a little heat, and economical. The last I bought was 22 dollars for 4 x 8 sheet. It makes no sense not to go with it, unless of course, someone already has whatever it is you've got. Below is a picture of one of my door panels that needed a little curve for my roadster. All it takes is a little heat from a heat gun as Dan said and you are done.
It sews easily and holds glue well. What more could you ask for?