FWIW -- I used a square-profile O-ring that I cut to make it the right diameter. Since it gets stuffed in there I didn't bother to glue the ends together but one could.
Thought I would buy a suitable O-ring but then found one left over from an old-style canister oil filter (they used to put a couple in with each filter).
I think you're right that the original would have been fibre, possibly red-brown in color, so an O-ring will not have the right look if that is important.