[IKE]

Polycarbonate.

We often use 0.020" through 1/16". This stuff is not cheap, but works great.

Some local sources include:

AIN Plastics in Madison Heights, MI
Total Plastics in Rochester, MI

McMasterCarr also carries it at a premium price but it is difficult to get large sheets.

We often use industrial velcro to attach the panels. Custom Vinyl graphics showcase well on this material.