Made a knob for my dimmer so may be useful for you as well. 

Outer diameter - 35mm. 

Stem diameter - 5.2mm.

Printed with tree supports (needed for inner dome, just to be safe) but with NO SUPPORTS for stem. If you want to keep it simple, you can also print it upside down (face to bed) but it may affect the rounded edges. 

STEP file included so you can change it for your needs!