Maybe the side wall of an old tire would work, punching them out with gasket punches. I’m just brainstorming!
The problem with tire rubber is its not the same as it used to be. I can remember using old tires on my car and they usually worked great until tread wore out. Now you get 5 years out of a set without them dry rotting or coming apart on you at highway speeds and your lucky. This type of rubber is also not happy with any kind of constant contact with petroleum products.
How about some large silicone orings?
Think I would agree with the leather idea.
