Aqualisa Control 164401

These two parts were created to replace the control knob on a Aqualisa shower, where the plastic pins inside broke. We are told this part is no longer on sale.

The small piece (the version with the slots) goes over the spigot of the shower and is held in place with a snap ring (7.7mm between inner jaws) 
There is an alternative version with no slots that you could use small screws to go though to the spigot to hold in place.
On our printer its a pretty tight fit and if you were careful you'd probably not need anything to hold it on.

The large knob is held in place both by friction and a small screw (M2/M3 your choice)

Printing
The small piece needs to be printed with a 0.25 nozzle. You can try it with a 0.4 but the 0.8mm teeth are so much better defined with the smaller nozzle.
I printed it with extra perimeters in PETG to give it a chance of lasting (I hope). 
I also used organic supports for the slit. 

The large piece needs to be printed with the large ring face to the bed and using supports.
This is a huge waste of filament (sorry) and makes the underside horrible. But we wanted to recreate the curved shape of the original, and we only needed to print one.
Using a height range modifier I switched from 0.2 layer height to
40.60-41.80   0.1mm
41.80-43.80    0.075mm

Our plan is to paint the knob with anti fungal bathroom paint as my experience is that unpainted 3D prints that are in wet environments breed mold (etc)

Alternatives.
There is also a round handle which we used as an emergency shower control until we finished the main one. 

Hope it helps. 

PS: This is the original
https://www.bathroomspareparts.co.uk/aqualisa-onoff-control-knob-164401-174522-p.asp