This is a collection of knobs i designed for M2.5 to M8 screws. I use them in my designs to adjust screws without needing tools.
Those knobs are designed for standard internal hex screws with cylinder head. The head dimensions are described in the table on the second picture. In my experience most internal hex screw follow those dimensions but you will occasionally find some who don't.
For each screw diameter you will find 3 sizes: S, M or L. It is designed so that S size would be as compact as possible, L size gives you plenty of strength to tighten the screw and M is a reasonable compromise i use by default.
Among the STL files you will also find a series of models called "TPU protection __ mm". Those are meant to help you grab the knob with a pair of pliers without damaging it when first inserting the screw like shown in the forth picture. It should be printed in TPU and you will need the biggest model bellow the outer diameter of the knob you are using. If you are not using many of those knobs a tissue paper will do the trick...
The knobs are designed to be a tight fit around the screw but i usually add a drop of cyanoacrylate glue to hold it secure.

A few years ago i published a collection of knobs (https://www.thingiverse.com/thing:5532943), but nowadays i often prefer to use knurled knobs as it has just as much grip and tends to be more compact.