I made a pair a while back during a blacksmithing class at the National Ornimental Metal Musem In Memphis. I had a buddy give me the critical measurments from his Gramercy holdfast he got from Joel at Tools for Woodworking. I based the shape on one that one of the smiths had made to use on his anvil. I used 3/4 round stock and the seem to hold just fine in 3/4 (+ a hair) holes on my bench. I would think 5/8 would work too. I think if your top is super thick 5/8 might be a better way to go.

Joe