Sorry for the delay but I'm having a terrible time with my internet [started writing this post at 11am, have had to wait 12 hrs to post it...]

The alliance hobby globe ring is a steel tube, 13m outside diameter, 1mm thick and 20mm long. The opening along one side is 4.5mm wide, but this can expand as you open it up.
Link here: https://alliancehobby.com/sights.htm