I'm afraid that cmstone has the only real answer here. Not what you want to hear of course. Build a new one and be sure it fits well during a dry fit. Drill holes a LITTLE oversize if needed as you can always fill with epoxy and let the excess squeeze out. Weight it down during the drying time to make sure it sets straight. Any attempt to straighten the one you have now will probably warp back at a later date.

BTW - The hard part may be deciding which side to replace.