I made my own wooden shift handle for my automatic. I don't recall the size hole I drilled in the bottom so you will have to measure again. I drilled the wood handle 1/32"or 1/64" size smaller than the shaft diameter for a starting point. Being wood, I was able to slide the handle on with some force. It is very tight and no additional fastening is needed. Just an idea for you to incorporate into your design.

James