Paperclay Fail

Unfortunately, despite priming first and weighting it afterwards, my attempt to use Paperclay to simulate plaster on the inside of the first shop resulted in three warped walls.  Weighting them now straightens them for a day or so, and then they slowly curl back up.  Paperclay and MDF clearly do not play well together.  (The surface is perfectly plasterlike after sanding, so it might have worked well on plywood ...) 

The top piece in this photo is the Plexiglass roof/ceiling, to compare something straight to the thicker back wall and the two thin side walls.  A number of ideas have been tossed around for bracing the walls with lengths of aluminum or brass, but I have come to the conclusion that the simplest thing is just to make new walls, at least the side ones.