London pattern handles to hold hex driver bits.  Two piece tapered thread collet style design.

The tapered threads compress the collet portion of the handle and squeeze down on the bit.  I tuned in the size of the collar so that it would get fully tight just as it closed the gap.  If your printer is tuned differently, this might not work out, so I included two other versions, “moderately tight” and “tight” .  The tight version is where the two parts are an exact fit to one another with no clearances or allowance for overextrusion. 

If you have to tune it further than these choices for some reason, you can just scale the collar a bit larger or smaller to fine tune the fit (x and y axis only) leave the z axis at a scale of 1.00 or the threads might not work properly.