I am building a shortscale bassguitar in the shape of the Ernie Ball St.Vincent signature guitars, and for this, I liked to also have the funky retro futuristic tri oval control knobs. So, I did a design in Creo, a ton of features, lots of surface modeling, as you can imagine.
I have had these resin 3d printed, in the larger of the two variants, but they both should be OK.
The way they work is that you insert an regular M3 nut in the slot, and fix the knob to the smooth 1/4" potmeter shaft using an M3 grub/set screw, a length of 4-5mm should be ok.

The photo is of a smaller version, with a letter embossed to show the function.