Well, while there's still no charging case for KO2, I'm planning to try just some tape–attached slim and light power bank. The challenge is just to find the most light, slim and narrow one, something like this:
https://www.amazon.com/dp/B00Z24G06M..._t1_B07CTH5T15
or this:
https://www.amazon.com/gp/product/B0...A3KN1WV8PHMYU5