This is a knob you can attach on hexagonal bolt head to easily turn it by hand. Written as a parametric module in OpenSCAD, you can chose the size of your bolt head. The perimeter of the knob has notches for better grip and the face of the knob has marks to track the knob position.

This is a parametric module for OpenScad. Load the module knob.scad file that contains the module and an example for producing two knobs for M10 and two knobs for M8 bolts (incidently what I need just now)into OpenSCAD. Modify the sizes to fit your requirements, generate as many copies as your need and export the .stl for printing. The knob.stl is the result of this OpenSCAD script.