siberglass alternatives

I have a friend who needs roof panels for a bus (like a cheeswagon) made and I have decided to help him out. The first thought that came to my head was to use fiberglass but this looks like it is going to be considerably expensive just for materials. I am wondering if anyone can think of something else that can conform to some serious curves and be painted? I see the fiberglass route as being a slippery and expensive road…

maybe ABS 4x8 ft panels? It wouldn’t be stuctural for sure though, and you would have to glue all the seams and rivet holes.

I’ve never messed around with anything like a roof on a bus before.