My Air-Bypass collar kept falling off so I used a 1.2 mm O-ring (19mm ID) and it works way better, once the collar is pushed onto the O-ring it's held firmly in place, almost too hard to turn, but I guess once a happy position is found it can just stay there. A 1.1 mm O-ring I think would be perfect but I couldn't find any. I'm keen to try out different coils soon, thanks @oddjobold for the info above.
Thanks. I like wasting money.
Thank me once I have tried it and it works!