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My take on the OP's problem:
1. Take your best guess, and then go 5x?
2. Strong enough magnets, no matter what protective layer you add, will likely scratch the paint by way of any dirt/sand/etc that collects on the, and then the strong magnet will nicely embed in the magnet cover. Of course keep the area COVID clean will reduce this issue when installing or removing.
3. And prudence IMO, would suggest a loose nylon? safety tether while testing. The forces to be calculated would have to as mention allow for odd wind gusts, from semis, etc, in addition, to rain reducing shear attachment, and road shock loads from roads aberrations/curbs/etc. So no matter what, you will have a big fudge factor, and the safety tether will help reduce that, IMO.
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