Using what looks like an insulated wiring clamp to secure the rod on the circle cutter is very clever.
Wing nuts with nylon inserts aren't very common, but are available if you would like to simplify the tool a little more.
I didn't have any suitable bearings when making my circle cutter, but did have some thick fender washers. One was cut and bored to size then held in place with a couple of spring pins through the rod. So far, spatter isn't much of an issue with it.