Should be able to fit in one of these cases: https://www.18650batterystore.com/collections/18650-battery-cases.
That said, can you just unscrew the tail a corner turn before putting it in your pocket.
You could put a slip of paper between the battery and the contact spring inside the light. New flashlights containing batteries are often shipped that way.
If you want to go fancier, Countycomm.com and Illumn.com both have various kinds of battery holders. Here are some particularly nice ones that are on the expensive side:
https://countycomm.com/products/delrin-aaa-battery-locker-waterproof
I have a couple of the countycomm delrin holders for 18650. I can vouch for both the quality and the cost. While pricey I haven't found/seen anything I like better. (3D print may be an option if you have a resource you can use)
Portable illumination
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