I guess it could be done with a dremel if you are really talented. I took sanpaper wrapped around a toilet paper roll and slowly opened it up... took about 10 hours total. Then I smoothed it upwith a dremel fine sandpaper bit. The hard part is the circle must grow to about 53mm without getting offset. It has to stay perfectly centered between the post holes. I probably hsould of had it machined but I'm cheap.