Thanks, Persist.
I’ve tried a couple of different adapter cables. The most recent were from KUNCAN:
They also make step up cables for 9, 15 and 18 volts. If you look around, you may find a single cable that will step up to different values.
This is the phone powerbank:
Mind you, I picked those two items because they were cheap. I doubt I will ever put myself in a position to truly need to rely on them and you may want to look around for more mainstream batteries such as the Ankers, which are also pricier.
I just bought a full sized BeatBuddy which probably has a larger power draw although at 20000 mAh you should be fine unless you are running a gazillion other pedals.
BTW, Rockabilly: what battery powered amp are you using?