[Arran]

Rich
 They used to use a similar material for making the inside door panels of cars, I think they called it "waterboard" or something. You can still get it from upholstery suppliers, though many upholsterers will use Masonite, thin plywood, or plastic instead since it they are readily available. If you have a laser cutter, and know how to use it, then it might be worth experimenting, I don't have one, and would not know how to use it so conventional wood working tools would be my method. In any event at the price charged for many of those radio backs, in many cases you could probably find a parts set with the back still on it for less, especially with plastic sets.
Regards
Arran