Home Depot supposedly has 4x8 foot panels reasonably priced. Use the 4mm thickness. Corrugated polypropylene copolymer sheets. I got mine from a local plastic supply house for about $10 a sheet.
https://www.copeplastics.com/coroplast.html A print shop may have some in stock or could order it for a premium. The hexcell you are referring to is probably more expensive to manufacture, but interesting.