My first attempt to model a knob for my woodworking jigs based on a knob I purchased a few years back.
I modeled it in FreeCAD which I am still learning and love a lot.
It is designed to accept a 1/4-20 t-nuts i got from Amazon 
https://www.amazon.com/dp/B07NCY2ZXZ?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1
They are a little longe than standard and seem to work well.