Batman Extruder Arrow, motor shaft has a slow taper, so should fit different extruder flow settings. Bigger at the mouth and slowly tapers down.

Edit: Did a test print, and the hole came out a little larger. But its tapered, you can sand down the bottom down a couple mm and make it fit snugger, but it all depends on what everyones resolution is like. I have my printer dialed in pretty good and tolerances are close.