I have a Model B with a PM5 that my wife and son use.
Concept2 no longer sells the Model B speed ring (gee, it's only been decades since the Model B's release, I can't imagine why they've moved on ).
Anyone know of a source for one - new or used - for purchase?
I think most folks just make one themselves out of cardboard or Coroplast, or whatever. Put it on at first with twist-ties and trim if you don't like the results. When you're happy if you want to attach it semi-permanently replace the twist-ties with zip-ties. If you look around the forum or the web in general you'll probably find a few examples.
Make one. That's what I did and it's easy to do. The outer diameter is IIRC 17" and the inner is the same diameter as the damper ring. You can use corrugated cardboard, poster board.... pretty much anything.