Eirther free hand it or make a plate from .125 and drill a 1/2" or 5/8" hole in it. Space the hole edge to piece edge to accomodate you joggle. 063 migth be enough depending upon the joggle height.
learning to free hand a countersink is an important skill. Its not that hard.
RV-6A / IO-320, Flying as of 8/2015
RV-10 in progress