This stuff?
http://http://www.conpearl.com/packa...-pearl-boards/
It's an interesting material. The middle layer they call pearls, I'd call egg-crating.
I'm looking forward to the Primary. I want to try slipping curved pieces of welding rod in the flutes to stiffen edges and get it to hold curves. And stuffing metal washers in slots to make attachment points.
Polypropelene isn't very stable dimensionally. There is a product called alumalite—
Alumalite | Alumalite Panel | Alumalite Sign Board | Laminators Inc.
—that is coroplast with aluminum skins on both sides. Good for strength, heat, and using adhesives. Spendy though.