[Joe RC51]

If you can't find a suitable O ring , you can make one out of silicone. You squeeze a bead out of the tube , in the appropriate diameter , into a bowl of water. The silicone will float on the surface and maintain the round profile it came out of the tip of the tube. Leave it floating until dry , then cut to length and glue the ends together with a dab of silicone and you have an O ring.