Those look really good. You dont need to buy mandrels with your engineering skills, just make a boxwood mandrel (either with a 2 mt taper or to fit in your chuck) that will fit into the inside of the ring with a cross cut on the end to the depth of the ring and use the live centre of the tailstock to expand it slightly, then you can shape the outside (you can even make stepped mandrels to handle multiple ring sizes) . For doing the inside just make a jam chuck again out of boxwood, to the depth of half the ring depth (if necessary you can cut a cross in the end to expand for a variable thickness ).