I seriously considered doing that from steel or aluminum and it
wouldn't be a problem with just one or two--but I want to make a batch
of 10-20 or more, and that would be too much work and a LOT of swarf.
I'd want a 1 1/2" hub, for easy turning access at the headstock end of
the work.

How about using weldable sprocket hubs and 1/4" plate?
--Glenn Lyford