What about doing something along the lines of the stage 8 locking fasteners? Where an extended point locks it in place.
Possibly make a rectangle out of sheet metal with tabs on each side. Then tighten down the nut and bend the side tabs up. As long as the extended side of the sheet metal tab will hit something to prevent spinning, then the nut shouldn't be able to spin either.
I dunno... it was the first idea that came to mind. lol.
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