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?
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
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.
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.