Freecad is a free software for 3D drawing : https://www.freecad.org/
It allowed me to draw a model that you can modify the inner diameter, the shape and the outer diameter. You will find them in the 3D files folder.
Here a screen shot of freecad, with the final result
To obtain the knob in real life, you need to find a 3d printer.
For example in a fablab near to you.
To install the knob on your potentiometer you need a screw :
I choose this one : https://www.bricovis.fr/produit-vis-sans-tete-hexagonale-creuse-bout-cuvette-acier-14-9-zingue-blanc-din-916-sthccuvzn/#STHCCUV03/004ZN
It's : M3X4 with Allen inside
1 - Global shape
To begin, we need to draw and dimension the overall shape of the knob.
2 - Revolution
The next step is a revolution around the y-axis
3 - Hole for nut cover
Create a cavity to hide the clamping nut
4 - Hole for the potentiometer axis
A second cavity must be created to accommodate the potentiometer stem.
5 - Knob grip
Create a groove to make the knob easier to grip
6 - circular repetition
circular repetition of the groove pattern around the knob
7 - Hole for the screw
Creating a hole for a screw to tighten the knob on the potentiometer stem
8 - Fillet
Make a fillet on the edge to soften the corners
9 - Add a mark to read the position
10 - Make a fillet to make the knob stronger on the inside
