How about drilling/countersinking and threading a hole in
the rod, then epoxy a flathead allen screw in?

Another solution is to face the rod, and drill three axial blind holes in
an equilateral pattern, just a little oversize of 1/8". Then
make a wrench with three holes 0.126", with short .125" dowel
pins inserted. Two-pin wrenches are more common, but
the three-pin variety has better stability...