If it's anything like the oil bath filter on my Cadillac, I think he's talking about the giant O-ring that goes around the perimeter of the air cleaner base on which the cover rests.
You might try making one out of soft rubber vacuum hose of the appropriate OD. It's hollow so it will compress.
Go to Home Depot or Lowes. They have tons of different types and sizes of hose for different applications.
