Its a big o-ring. I had trouble finding one but I did find one that was a little to small. It stretched like a rubber band and fit in the groove and works like a charm.
The groove the o-ring goes in is 4.25" in diameter. The thickness of the o-ring is about 1/8", or just a teeny bit less but I bet 1/8" will work fine.
I use o-ring number: 2-155 N-70 It is a little small and has to be stretched on but it works just fine. It was the biggest one the place I went to had so I was stuck but it was cheap and I got it right then and there -- no waiting.