In my jeans little watch pocket, I carry one of these guys:
These are cool cases that are really close-fitting for one 18650 or two 18350's. The case is made of Polyethylene Teraphthalate Glycol (PETG) so It's rigid by nature, but having some certain degree of flex to them - particularly in the molded caps which feels silicone-like. That's why it is somewhat comfortable to carry in the watch pocket. Not sure if it's totally waterproof though, but the cap does come down substantially along the length of the square tube. Splash-resistant/weatherproof maybe, but still insulates the content well enough from any metallic object.
I got them from Jason of Dark Sucks as he provides them FOC with purchase of 18650 from his store but he sells them loose too, custom ordered from Caplugs.com
Jason's Maker's sticker was placed by me though, so I know which 18650 to go with the Prometheus Alpha that I EDC.
Sent from my iPhone using Tapatalk, please pardon typos.