I polished the edges of some 100-deg tinnerman washers on the 3M wheel and tested the fit with high-tensile 90-deg #6 socket head machine screws and they work out just fine.
As #6 aviation screws don't come in structural grade, I chose to go with screws I clould reliably remove from the nutplates beneath without stripping a phillips head socket. They will need to be painted to control rust, but that's OK.
