If you guys don't end up making one and can wait a month or two, I can make some. I'm still finishing up the remodel but I was considering this as the "next project". I need to build a new door for my barn anyhow so I could just get some extra ply or something.
I was thinking of laminating luan over a stiffer substrate, sanding and then painting. Luan is naturally smooth and easy to sand to a 64 finish, so that then a couple coats of some nitrocellulose lacquer for a smooth, slick finish to protect the paint.