Two 36"x42", oil on wood panel portraits recently sold (very happy to say goodbye)! I couldn't send them off without fixing the warped curvature that made them unable to hang or lay flat. In the past, I attached an extra wire to two screws in the offending corners and twisted tight, which eventually was useless. New simple steps: corner brackets, and a 50" screen door turnbuckle.
Corner brackets in place. This one needed a third hole drilled into center to accamodate the length of the turnbuckle. S hook used to connect.
This worked well for a 42" painting; different sizes are available, or you may have to cut the flat end of the rod with metal clippers where it makes sense. The size/length of the S hook (or other clip) will also make a difference. I think these are 25mm. Tighten in center until the panel is laying flat.