Just did this today, had a bit of problem opening the case, but got to it eventually after trying different tricks
The rings are very good quality. Got them on Ebay germany for 29 euro. They are not from plastic, but solid aluminium. When I mounted them, they clicked in right away, but they didn't stay 'rock solid' in place. You could still turn them around a bit inside the plastic holes of the casing. But once you put the 2 parts of the tacho housing together, they got stuck and nothing rattled at all. Hopefully you can all understand what I'm saying here