Okay, let's talk about Battery Storage. You may know that I have several Storacell Caddies in the house and car. I also have a THYRM CellVault XL clipped onto the back of my backpack. The clip is threaded through 2 MOLLE loops. It takes up very little room and doesn't weigh very much.
There are several models of CellVaults -- the original, the XL that's twice as wide and holds twice as many batteries, a large modular system, one specifically for an 18650 or pair of 18350 batteries, and one just for 21700 batteries. More of the newer lights are shifting toward that size because the capacity is so much more than an 18650 while barely being any larger. That's what my 3,100 lumen Olight Seeker 4 uses. An 18650 would fit in the 217000 holder with 3mm side to side play, 2/3 the diameter of a BB. And there would be 5mm extra room end to end if it wasn't made to hold 2 18350s, the same length as a 21700. I think I would buy that one over the 18650 size even if I wasn't currently using 21700 batteries, just to allow for a size upgrade.