If you've got a broaching set-up hex is easy, if you've got a CNC machine hex is definitely possible. I don't want to trust a grub screw and I don't really have a shaft to put a keyway in.

The GR02 doesn't have a long shaft, especially when you have 3mm of aluminium bulkhead and a bearing block to go though. I wanted to use the Banebots hub to extend the shaft as well as allow me to use the same wheel & pulley setup on both driver wheels and driven wheels.

Also HPC gears seems to think a 28 tooth, HTD 3mm pitch, 9mm wide belt is okay up to 134 Watts, which is 105 RPM at the GR02's rated stall torque of 12.1Nm.