Summary

I changed two things,
first) I added a parameter so that the shaft sizes on the two ends can be different.
second) I changed to using a 'pinch ring' for mouting to the shaft.

In this case the pinch ring is a Nut that I have drilled the threads out of, and tapped a screw into the side of. Iprefer this as it puts compressive load on teh plastic and dosn't try to stretch it. So far I'v had really good success with this method.