Masonite: A Pain in the Tuchus

I got a bunch of panels of masonite on my last run for materials and a showerhead at Home Depot, and it's warping, even though I've put a couple of coats of gesso and/or acrylic medium on 'em. I don't want to sell warped pictures. I think I'm gonna have to box 'em like I did for Batmonkey:


Annoying.

Any suggestions?