Topic: 1967 Pop open gas cap gasket. Cap to filler tube reproduction?

[1967 eight barrel]

I had my original Shelby pop open cap replated. The gasket doesn't seem to seal well. I found this gasket in the NPD catalog. Has anyone used this on the pop-open cap? What is the material?
                                                                   -Keith
https://www.npdlink.com/product/gasket-fuel-cap-repro-on-cap-to-filler/106665?backurl=search%2Fproducts%3Fsearch_terms%3Dgas%252Bcap%252Bseal%26top_parent%3D200001%26year%3D1967&year=1967