Plastic fluteboard is very much like corrugated cardboard, but made from plastic. It can be found at sign companies that print custom signs like for the politicians. Large sheets can be found at plastic dealers. Coroplast is one brand name. It is not hard to find, but will not likely be at your local big-box home improvement store. Note that using this material, is much harder to make a solar cooker that will fold down compactly than when constructing with cardboard.
One way to address the folding problem is with the use of plastic zip-ties. The Panel-Box Cooker uses the zip-ties to create hinge connections for the panels. Fluteboard can be creased and folded, but it does not offer the ability to have the articulated panels lay flat for storage or shipping of the cooker.