With all the great machinists and expertise on this site, I hesitated to post my simple tool. However, here it is for those like me who lost theirs.
3/16x3/4 woodruff key
3/8x2 union
File a 1/8 deep slot across the face of the union, just wide enough to drive the key into. Solder or JB weld into place.
Bobby
PS: Posted over in the Centerfire section, just though I'd drop it here in the right place.