50 years ago my wife painted a picture on stretched canvas over a
commercial art stretcher. The stretcher is 40X30". The stretcher is
made of "thin" 1X2" The original frame did not add any structural
support to the stretcher.

Over the last 50 years it has developed a bend in the plane of the
diagonal between the corners. ie this meant that one corner was about
3" from the wall when the other three corners were touching the wall

I thought it would be an easy fix with a cross made of 1x2" material
that fit a cross the back of the stretcher. The arms cross at 90
degrees, so the arms are attached to the stretcher about 6" from each
end of the stretcher.

This took out a lot of the bend but still left enough so that the one
corner is about 1" from the wall when the other three corners are
touching the wall.

Is there any other way to get the leverage to take out the rest of this
bend.
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2018: The year we learn to play the great game of Euchre