OK, so IDK if the size will be the diameter across the flats or notâ€¦practically to figure the size you might have to make one out of balsa wood and glue to see what fits right. However, once you get the size, if the inside diameter across the flats is what you use to compute the size, just add the thickness of the material times two to get the outside diameter across the flats. Once you have that dimension, call it D, then the side of the hexagon, say S, is given by D/1.73=S. Once you have the side length (S), just multiply by six to get the length of stock you need. Use dividers to mark out the length of the sides and file as appropriate and you should be there. You might want to use slightly larger stock than originally contemplated so that you can stretch the ring if it is too smallâ€¦you could also just use an emery and grind off some of the inside circumference, I guess. At any rate, this is what I get from geometry formulas that Google gives me. -royjohn