Sorry for the late answer, travelling in Europe. I am very happy with HDPE. Absolute zero issues.

George, I hope all is well. I am currently building a Transit and unhappy with the 1/4 baltic birch plywood and would like to switch to HDPE. You seem to be the only person on the interweb that I can find that has used it. Are you happy with it's performance and does it have any off gassing or strong odors when it gets hot in the summer? Thanks for any information. Brad