I have used one of the "toy" caribiners for a belt key holder for years. You can buy they anywhere for a few bucks, and sometimes public agencies give them away for free, with some safety wording on them. Your federal highway safety tax dollars at work.
I do not trust just clipping the 'biner to a belt loop as some do, I run my belt thru the 'biner, then clip the key ring onto that. And I have my keys (and I carry quite a few, plus other little goodies with them) on a pretty large split ring, about 1.5" in diameter. Makes it easier to clip onto the 'biner...
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OBG