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Looking at your picture of your boxing, I notice that you have a stress concentration that you should address.
At the red arrow, you have a bracket for mounting something on the top of the frame, and your boxing plate is cut at right angles to the frame flanges. That makes a spot where the frame goes from stiff to flexible suddenly. The boxing plate should taper toward the flanges to add some flexibility where the boxing plate stops. I drew in a yellow outline where the edge of the boxing plate should be.