HOW did you snap the shaft?!?! I've never even heard of that problem.
We've never even bent one. What weight is it? And do you run the timing belt inverted to let it slip on hits?
Either you had the belt way to tight... Or some other stuff that you did contributed because I can't imagine that happening.
You should just be able to make a slight flat spot or two on the motor shaft and then use 2 set screws at a right angle on it to hold the insert. We've used a similar method for all of our robots and we never had an issue in the aspect you're speaking of.