Simple cap for 18650 battery to protect end(s) when carried loose, reducing risk of a short. Fit one at both ends or just the positive. Could also be used to track charged and flat batteries. You might find the plastic film sleeve on your batteries is a tiny bit thicker so I've included the Tinkercad model link to make the centre hole larger. I'd suggest only making it 0.1mm bigger and trying for fit.

https://www.tinkercad.com/things/1Gdh5gR7rCq-18650-cap