I forgot to mention that a car sized deep cycle marine battery would probably work fine, as they are a lot larger than the batteries in those jump boxes. I’ve used one when needed for years, for charging 2s batteries and the occasional solder job, typically with 3 chargers or so. Can’t remember how many batteries it can do without recharging though.

Generators are cool though, except for the gas as you mentioned. Just don't be that guy with the noisy one!