NOTE: I have just updated and re-uploaded the model as for some reason it was in meters instead of millimeters. Thanks Areeve20 for pointing that out! Please download the new files!
Print the cap, buy TWO D2H1 magnets, mount magnet, use as a screw driver/ sharpie cap.
( I bought mine here: kjmagnetics.com/proddetail.asp?prod=D2H1 )
More detailed instructions:
Stick the magnets onto the flat base of a screw driver bit, and press the whole thing down into the hex-holder on the cap, this will force the magnet down into a tapered slot. Once its inserted into the slot, drop some super glue from the other side of the cap (The side that the sharpie goes into) to glue the magnet in place. Once dry just pop in your favorite bit and drive screws... or un-drive screws.
Note I have included a quick video, an .STL file, and an .IGES in case anyone wanted to make changes.