If we went ahead and tapped the hole, filing a stop was part of the deal. An adapter could be milled, problem is that you are probably going to be limited to a 5 or Ultra 6. As I am writing this one concern would be making sure there is enough thread engagement into the dropout, the thickness at the swing zone will probably have to be 1.5 mm at most. I would probably start with 1/4" alloy plate. And not do it without access to a Bridgeport.