The idea is to use a Dremel tool to cut out a small cavity in the motor shaft that the grub screw could latch into. The cavity does not have to be that big, but it has to be wide enough to fit the entire grub screw head into it.
I began with removing the extruder motor and removing the 9-tooth gear.
This is the tip I used for the Dremel tool.
With the motor secured in a vice, I could start making the cavity.
It took a bit of time and came out a little rough, but it does work. I would have preferred a cleaner cut, but this is the best I could do by hand.
Now I don't have to over-tighten the grub screw to get a good grip on the motor shaft. The extruder works much better with this simple modification.