You should try a "9-speed" ring. When I converted one of our tandems from 9- to 10-speed, I left the original 9-speed front derailleur, crank and (53/42/28) chainrings on the bike just to see what would happen. Works as well or better than ever (and I'm quite particular about how our bikes work) so I left well-enough alone.

[FWIW, the 53 and 42t rings are Shimano Ultegra; the 28t is third party -- could be Salsa, for all I know.]