Coroplast:

Description: A tough, rigid, inert, and chemically stable corrugated plastic made from a copolymer of polypropylene and polyethylene. It’s resistant to water, oils, and solvents. Coroplast can be formed into boxes suitable for storage and transit. It is an excellent backing material for paintings and framed works on paper. Due to its water- and puncture-resistant qualities, it is also used to cover the open space in transit frames and hybrid crates.

Sizes: 48″ x 96″ x 4mil. Available in natural (transluscent) or white (opaque).