I'm using a bit of inner tube I saved from an old tire I removed. Butyl rubber can be cut to match the flange to insulator gasket, should be fine in this situation and will not lose vacuum. Ask any tire shop to save you an old inner tube. They're not as common as they were 30 years ago, but people still use them. I need to remove another old tire and will see if there's a tube in it. You're welcome to some of this if you want it.
Jon


~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end