Knobs for 1/4" bolts! Diameter of pictured knob is about 40mm (~1.5 inches).
Once printed you will need to drill out the small hole the bolt goes through with a 1/4" drill bit as it's modeled a bit undersized. I do this on purpose and then always drill the hole out so it's super clean/round.

You can put either a 1/4" bolt or a 1/4" nut into the knob depending on your project design.

Tip: Epoxy the bolt (or nut) in place if the knob will be regularly completely unscrewed so you don't loose your bolt (or nut).

The file al_KNOB.stl was made in Onshape (See photo of knob printed in yellow ABS.)
The other two (old) versions were made in FreeCAD. One of those stl files also has a spacer piece that was to move the knob further out from the surface it was screwed up against.