You found the solution!
The object that the U clamp…clamps around, must be thicker or else the screws will be subjected to the torque from the pitching moment.
The best option is to move the L beams closer to the max thickness of the U clamp, and perhaps put something in between them to pick up the clamping force. But the L beams look like they can take it.
Making small dimples, with a drill, in the L beams for the screws to dig into will also help securing the base.
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