Fan that slides onto the motor shaft. Standard is for 5mm shafts as my Lindinger Planet Hobby 3530, but I have included an untested version for 4mm shafts too.
It is very tight, so you maybe have to use a file or a drill. I didn't want to make it loose, so this is intentional.
use M3 grubscrew to tighten
I have used a (it seems now defunct) openscad turbine generator for the fan and added a shroud and m3 grubscrew hole

you can find the car on https://www.3dsets.com