First was looking at some photos of real ones to see where to drill the hole on the mule.
I used a mini hand drill to do this, as more control, can't remember the exact size of drill bit used, somewhere between a 4 mm to 6mm I think.
Finished the look with a hollow metal rivet, of the sort you might use in d.iy. glued into position, dab of paint. That soon wears away as you turn the RR.
Also had to file the fixing numb a little to fit rivet. But you could miss out the rivet as not really noticeable. If doing that, drill a smaller hole.
Hope this helps Mike.