Blackboard was available some years ago in the US. It is a reasonably stiff rubber sheeting that could be used as replacement firewall, rumble seat side panel liner etc. It was covered by the upholstery of your choice. The sheets were 3/16" to 1/4" thick and would be easily bent and hold a shape with minimal attachment.
Is there a source for this material ????
If you "google" LeBaron Bonney, you will see that they carry the panel material. They have a 1 800 number so you can contact them. I could not remember their name before, but Will remembered where we bought the paneling we used.