You really couldn't go wrong with an Anker. This is one of their newest.
Anker PowerCore II Slim 10000 Ultra Slim Power Bank, Upgraded PowerIQ 2.0 (up to 18W Output), Fast Charge for iPhone, Samsung Galaxy and More (Black) https://www.amazon.com/dp/B01NBQX0O5/ref=cm_sw_r_cp_apa_v4lJAbFZC8NPN
I have this one which fits along the side of the headset.