All the Vive headsets require 12V so that's met with the qualcomm quick charge 3.0 requirement. Anker quick charge 3.0 batteries tend to be very reliable and are easy to find. The Anker Powercore Speed qc 3.0 power banks even support up to 24Watts (12v at 2amps) so should work with Vive Cosmos as well as OG Vive and Vive Pro. The Anker Powercore QC 3.0 (without the Speed in the name) support up to 18Watts (12v at 1.5Amps) which seems to be more common for quick charge 3.0 batteries. The Anker 10000 batteries are about the same size as the HTC ones. The 20000 ones are longer.
This one will fit nicely in the belt clip: https://smile.amazon.com/dp/B01JIYWUBA/ref=dp_cr_wdg_tit_nw_mr
This one will fit but stick out of the top: https://smile.amazon.com/dp/B01N0X3NL5/ref=dp_cr_wdg_tit_nw_ls