I picked up a lot of 3 modern but warped 7x17 sheet film holders. The lot was less than 1/3 the price of a single new (but unwarped one) so I thought "what the heck?" In the dryness of the desert the least-warped one returned to flatness, but the other two didn't. The better one is off by 5mm at the opposite corner and the other by 9mm.
I was thinking of clamping them to a metal plate and using moist heat to see what effect that had (putting them in an oven with some pots of water, at maybe 80C or so). After that I planned to rout a 0.375 x 0.375 recess down the sides on the holders (while still clamped to the plate), stopping at the light trap and flap areas. Then I was going to epoxy an aluminum channel into the recess.
I don't know if the channel will have sufficient stiffness to keep the holder flat, or maybe I need to slightly bend and preload the channel to keep the holder flat. Or is there a better way entirely?