I would advise using a 20mm silver steel shaft and some bearings either end machined into bulkheads. Make the length of the shaft as short as it needs to be. I've discussed with Mario the merits of bushings, he is a fan, I'm not. I'm not going to comment further on that one.

Material wise, NST uses ali (around the 10mm mark if I remember right) however a steel construction or even a well designed thick nylon bulkhead could work well.

I would recommend against mild steel but if you are set on it then make the teeth from tool steel or similar, key them to the bar and bolt on. Easy enough to get a hold of a thick piece of Hardox which will do the job well, contact JL steel services who will be able to plasma cut your design for you (you will need to tidy it up with a grinder or mill).