I know what your saying Roger, trouble is I've got 8 XAircraft motors on this thing which obviously have the flat bits on the shaft, so dont know what else to use without changing out 8 perfectly good motors.
As I see it, the actual prop retention and locking principle is based on prop compression to the motor base plate and not with the flat key on the shaft. If it was the case, how to explain all the other and more powerful motors featuring no flat portion on the motor shaft.
Also, the electronics of all Master Controller (MC) that I know feature a so called ''Soft Start'' which prevents any abrupt loads on initial start up that would affect the solidity of the prop spinning off the shaft. Having said this, nothing that I can see prevent you from getting 8 new APC props, at 2 - 3 bucks each, using the prop hole adaptor (included) if necessary and applying the same torque as usual to securely compress the washer onto the prop!
Here you go, flying safe with peace of mind and for really cheap! (compare to the other alternative)...
Now, I do not fly XAircraft MR nor do I use XAircraft motors but looking carefully at it the standard DJI motor (2212, 920 KV) I noticed it has a FLAT on the shaft TOO! As hundreds of people fly these motors with APC and Graupner props without problem (including myself), why not using those props on you motors?!