Replacement cap for JosephJoseph Y-Grinder

EDIT: April 2021 it broke again. Added a knurled version, because some filaments are quite slippery and difficult to grip when printed as a smooth cylinder.

The JosephJoseph Y-Grinder is a twin-chamber salt and pepper mill: http://www.josephjoseph.com/en-gb/product/y-grinder/

I have had one for several years but through prolonged use, I have managed to break one of the caps, which means it no longer securely grips the splines on the mill shaft.

The splined shaft is a bit tricky to m