Hi Mike,
You can use a plastic called HIPS. (High Impact Polystyrene Sheet)
Here is the Materials Data Sheet on it from a company called
City Plastics
You could probably buy it from them. I bought it from a company in Chatswood called
All Plastics Engineering. It cost me about $15 for a sheet 1.2 metres x 1.2 metres x .7mm thick. Its very strong and light and can be easily attached with self adhesive velcro.
CS-John B