Could this solution work if the mast on one side a bit moves in the wind. Distance 660m?

Why would a mast move??
The whole idea is to attach the cube to a stable platform be it a wall or a mast.
The question is how much vibration are we talking can be tolerated (degrees or some measurement of the range of motion at the cube).

I could take measurements if I knew the beam angle of both frequencies. Is there such information?