I’m familiar with the felt gasket the OP is describing, the “heavy duty” oil bath I put on my truck (because it was the easiest way to convert to paper) has it.
It sits in a little channel between the two metal layers that sort of sandwich the round part of the airhorn. Mine was torn up and unusable. Experimented with half a dozen materials to replicate it and never did find something I was totally satisfied with. In fact, I’ve forgotten what I ended up using. Maybe a little piece of soft rubber tubing?
I think a square profile o ring of a very specific dimension would be just about perfect. But I think I got fed up before I ever found one.
