These are copies of shutter stops. The originals were affixed by wood srews which ripped out. The are installed on powered shutters, so a friend asked me to make him some. The flats allow for a big contact area/low pressure on the part, even if used on powered shutters. 

Any machine hardware is plenty strong to withstand anything. I recommend M5 nylon lock nuts of the stainless variety, since it's for outside use.

V1.0 is a direct copy of the outside shape

v1.1 has a slight taper where the flat meets the base to counter notch effects

Designed on Tinkercad.