John1, attached is a photo of the O ring I ordered on eBay.
I tried using scissors to trim one of those foam Air-Tite T-21mm rings down to 24mm O.D., and the result is functional but very uneven, to put it mildly. I'm not sure any custom work I would try on anything else would work out any better.
I may just avoid using an O ring to be safe, even on a circulating nickel. Apparently nitrile is relatively inert compared to regular rubber. I found this online: "Nitrile rubber is used in a wide variety of applications, such as O-rings, gaskets, oil seals, automotive transmission belts, hoses, V belts, synthetic leather, disposable non-latex gloves, printer's form rollers and cable jacketing." But of course there's nothing in there about its potential interaction with copper and nickel.
Thanks for everyone's comments.