Check Home Depot for a 4x8 sheet of Luan in the plywood section. It is thin and strong. Can be covered w material or stained/coated to use the figure of the wood.
Or a sheet of Coroplast - white corrugated plastic could also work. Can be used as is w lighting behind, or covered w material.
3M 90 spray adhesive is good for adhering material to either.
Both materials bend a bit to conform to the curves of the roof or walls and are light weight.
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV Mid Top, Custom Walk Through, Lots of stuff added. BlingMyRig.com