Does anyone have recommendations for waterproof case that are sized to fit 1 18650 cell or 1 21700 cell?
The older links for the 18650 (Waterproof single 18650 / 2x 18350 case) and (Waterproof cell carry 16340/18650) seem to be mostly 404ing, from Aliexpress and Amazon. I assume that’s because they’re from 2015 and other products have came out since.
For 18650, the Soshine stuff is still available from their storefront:
There is also these that look similar:
I have no experience with either, just sharing. Single cell cases I could not find, 21700 neither.
Also try over HERE. He’s a member of BLF.
Oh, that’s a fun design, but I’m not sure if I trust 3d print for waterproofing, even with a gasket.
Those Chinese ones look like crap
I prefer these
I just wish they made 5 x 21700 case too or 8 x 18650
Thanks and I agree with your points. I noticed most linked stuff only have 18650 as their primary focus for casing. I assume it’s because 21700 as mature when people looked into it?
These match safes fit 18650 singles. They used to be around a dollar or two at walmart here iin the camping department.
Soda Bottle Blanks, aka soda bottle preforms, aka baby soda bottles, will hold two 18650s in a test tube looking thing.
Generally, about a dollar each in quantities of 6-30, but some sellers will charge a lot more. Sometimes you can find colors.
Amazon has them as [Steve Spangler’s Large Plastic Test Tubes], as well as a bunch of other kinds
On Ali there are short alu pill-boxes like that. I use those to keep 18350 and CR123A dry.
How do you isolate them? I’d feel uncomfortable having LiIon cells in a full metal box.
Test tubes are actually a good idea. 50ml test tubes are dirt cheap on AliExpress, definitely waterproof, and should be ~80x26mm ID. Hard to tell since the bottoms are pointy and they only give outer dimensions. With some padding, that would work.
The best waterproof case for a spare battery is another flashlight that uses the same battery. Preferably one that performs a different task than the first flashlight you are carrying.
Ideally one that has USB charging too, so the spare case doubles as a spare flashlight and triples as a travel charger 
That depends a bit on the size of the pillbox. The total length of my boxes is 52.3mm (2.06").
The containing part is 40.6mm long (1.6"). With an outside/inside diameter of 20.6/19,1mm (0.81/0.75").
The trick is that the cap of the pillbox is holding down the battery, without touching the positive pole.
Wrapping the battery with a little bit of paper prevents if from rattling.
I’m using it to store CR123A and 18350 cell in my backpack. Maybe I’m just lucky to find the right size of pillbox.
Sorry for goofing up, but this was meant to be the answer to @ebastler
I have carried soda bottle blanks filled with batteries while diving. They work great. I also like the small Pelican Micro cases.
You can’t go wrong with either.
These are the best ones I’ve found. They are expensive for sure, but I’ve had good luck using them on multi-week kayak trips.
