The cardboard is called "hat rack board". It is sold to replace the panel behind the rear seat on cars. It's a 1/8" thick cardboard with a textured finish on one side. They come in 4' X 8' sheets and are available at auto trim & upholstery places in 4 or 5 different colors, and can be painted to match the original. They can be cut into any shape easily with a pair of good tin snips. I used it to replace the originals on my '59 panel, and they look great.
