Just make yourself an inserted cutter, even if you don't hit the width, you can take a couple of passes. It's not that critical anyway. Nothing bears on the 45°'s.
The only reason I see for the angled shoulder is to make sure (help to) the set screw doesn't get hung up on the 90° shoulder of the cut.Is there such a thing as an end mill that cuts 45° weldon flats? Can an existing end mill be ground to cut weldon flats? It's not easy to cut two 45° angles with a flat in between all the same depth.