Original Amp-meter Restoration

[29er]

A while ago there were a few threads about obtaining reliable amp-meters. I've had a couple of repops that were less than satisfactory, so I purchased an original from Bert's and am cleaning it up.

Inside there's a sort of gasket that's round in diameter and holds in the plate to which the needle attaches (this is not my photo, but one I lifted from another thread):



To complete my restoration, I need to remove the original gasket and replace it with something.

But what? What was the original made of and what can be used to replace it?

Many thanks!