Made three changes but kept overall dimensions:

- thinner shaft as stock was thick enough to rub on the rotor during operation
- Angular keyed cutouts for the stepper motor shaft to fit into, which should stop spinning in place.
- better fillets as the stock ones were kinda abrupt and ugly (!) this should also strengthen the transition areas between slopes. 

TIP: Print One test first to see if you need to scale model up/down in your slicer to get a good fit. My prints as-is with a little force to get on and a satisfying "pop" when you pull off.