[JTW]

You can get upholstery cardboard and cover (glued on) it with black vinyl (sewing area in walmart etc.), this what the professional shops are doing. The problem with the cardboard available is that after time and moisture it gets wavey. I have used 1/8" plywood before and 1/8" insulation board before and both worked and both were able to be sewn through if you decide to put a binding like the original