Cheaper quality lockrings are often thicker because of lower quality metallurgy.Had exactly the same trying to use a miche cassette. As some others have said, the locking ring threads were too long and fouled the dust cap. Shorter shimano lock ring sorted it.
Thanks a lot ishaw, I think anything 12T and up. With the allowance for the lip or step on the freehub.What cog ratio are you after? I have a couple lurking, ones a new Shimano, not xt though. I remember that some Shimano ones seemed to have a recess in the alloy carrier which may be to account for the step at the back of the freehub. Not sure which Shimano designation was used but they used 'type' and a letter alongside the model. Some came with a spacer to fill the gap, maybe to make it compatible with other freehubs?