Material Source-- Gasket for Fuel Tank

I need a gasket to fit the auxiliary fuel tank fo an Aeronca 11 AC. The existing gasket is partially decomposed and broken. The caps for both main and auxiliary tanks appear to be identical as, of course, are the gaskets ;probably a neoprene- like material. Without thoroughly "vetting" the owner maintenance aspects of this, I think reinstallation of the gasket is probably simply to slip the replacement over the fastener teeth of the cap. Have checked with Aircraft Spruce but couldn't find anything. so need a source. I can easily fabricate the gasket from a suitable material but would go with an actual part if possible. The AUX tank is not being currently used but just trying to catch up on overall condition of the Chief.