I made my MkIII's side cover "bracket" out of a standard grab rail clip (063798) which I mounted onto the frame tube, and I used an aftermarket horn bracket that just happened to be lying around without a job, for the arm of the bracket.

I drilled three holes in the bracket, the centre hole I filed to a rectangular shape to accept the standard (066307) Dzus "receptacle", which I fixed in position with two self-tapping screws.
I found that the assembly does need to be set so that it can move vertically by a small amount, so that the end of the Dzus fastener "stud" (066435) clears the receptacle when the panel is removed/refitted, and for that, I used a nylock nut not fully tightened on the clip bolt/bracket fitting to tension the clip, so that it will move when the bracket is pushed.

So far, I've not had the panel come loose anyway.