Customizable Knurled thumbscrew cap for machine screws. Simply glue the cap to the screw head with some epoxy or superglue.
Design includes some internal indentation of the screw drive so that the cap grips better while being torqued. If it's a standard phillips head, the defaults params should be OK.
I would recommend filing/sanding the screw head a bit as well, to give the glue more to stick to.
Uses aubenc knurled finish library: http://www.thingiverse.com/thing:31122