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Grab a (shimano) cassette lockring and try that instead.
I'd go for the disc lockring because some of the Shimano cassette lockring have a wider thread, some are made from alu, etc.
Ask around, some shops should have those steel one lying around, I know I do in my workshop, this photo clearly see how much less it stick out;
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Thanks for finding a picture, I was struggling on my phone! I wasn't suggesting a cassette lockring as a permanent solution, just to check the fit. The being said, I would have thought alu would be ok, it's seeing the same type of loading as a cassette due to the splines. I guess it's the safety critical nature that makes steel preferable.
Shimano center-lock lockrings are the same as for their cassettes. Hunt have provided a DT Swiss style one that plays nice with thru-axles, but that doesn't seem to be an issue for you here. Grab a (shimano) cassette lockring and try that instead.