Started with 3/8x1-1/2 SSTL fender washers and opened up the ID to 9/16". I made a drawing on my computer, printed it out, and then tapped it to a piece of wood. I stacked the washers and bolted them in place with a bolt. I mounted the jig in my drill press and proceeded to drill three mounting holes 120 deg apart. Drilling the mounting holes with the washers stacked ensures the two pieces are identical. I decided not to countersink the holes. Tomorrow I'll sand them with 400 and then 600 grit wet sandpaper for a brushed look