1965 Dart Horn Emblem O-Ring?

More horn questions today...

It looks like the center horn "emblem" that fits into the horn ring is held snug by an o-ring (according to the manual). The one on mine isn't big enough to hold the piece snug, and I think a prior owner just glued it in. Anyone try something off the plumbing shelf or at the hardware store? Am I barking up the wrong tree here? :) Thanks!