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It seems like your printer not quite fine-tuned. I printed a new ring (previous I gave away) and measured it. It has inner diameter 54,10 - 54,40 mm, while the corresponding diameter of the lens 53,90 mm. Despite some ring ovality and a slightly smaller size than specified from design (54,6 mm), it is well fit to the lens.
If you can't insert one ring to another, this may indicate a certain ovality of the ring.