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Resin? I use hot water, a piece of glass and clamps, the fridge and room temp.

Place the warped part in very hot water and leave in the water submerged until you think the heat has penetrated completely. That is important, the core must be heated.
Clamp the piece securely to the sheet of glass, the retained heat will allow you to do this easily and without cracking the hull. Put it in the fridge (not freezer) and leave for a day. Remove and then let sit in ambience until no longer cold. Unclamp and straight as a die, and you have permanently removed the warp memory.

Take all necessary care when doing this.