There has been some interest in this tool so I have made a sketch of it. I scribed a 13/16" circle and a 5/8" square for the hole. Then I drilled holes in the four corners and then the center. Then it was whittling the flats into the scribed circle with a 3/8" round coarse file and finishing it with a 3-sided one. I often checked the shape with the head of the one bolt I was able to remove. When it was pretty close I pressed the head into the hole to get the right fit. I believe these are correct dimensions but check them as you go. Some grinding on corners may be needed to get the proper fit.