Backyard Aquaponics
A place to discuss aquaponics

If I use a solid piece of tubing and drill out the center, I could then heat it and fit it over the larger shaft.

Possible, but the small flex that you would get out of flexible tubing could prolong the life of your motor by taking some of the stress off of it. For example if there was a minor jam. The flexible tubing would "load up" like a spring, hopefully forcing the jam free before putting too much added stress on you motor.