Now we’re on the topic off Li-ion temps anyway… 
I also wonder to which extend battery adapters hinders batteries in flashlights to get rid of their heat. For example on left of the photo below is a 26650 > 21700 adapter that came with one of my (26650) Astrolux flashlights. It basicly wraps the whole side of a 21700 with a 2,4mm thick ‘plastic’ sleeve, which I can’t imagine is beneficial for the temps of the battery.
On the right of the ‘plastic adapter’ is what Simon/Convoy sells as 26650 > 21700 adapters. Although ‘air’ also isn’t the best conductor of heat, I like to think that this way there is less heat trapped by the ‘adapter’ than in comparison with the (thick) plastic sleeve.
So that is maybe also something to consider if you are running a battery in combination with an adapter in a (single cell) ‘High Lumen/FET’-flashlight.
