How to fabricate panels

I need to remake my package tray board and the pieces that go between the seat and the back edge of the rear doors. These are two layers so the panel clips can slide between them to secure them to the body. What backing material would I use to make these? The package tray is one layer and held in place using two spring button clips. There is also a piece between the package tray and the top of the seat I need to remake. Any suggestions are welcome. The originals are still there but in bad shape. I would like to duplicate them before they completely rot away.