Several of the newer Chromebooks support USB-C charging, allowing a number of external USB-C PD (PD = power delivery) batteries to do what you want. You certainly get a monster like the Anker PowerHouse that supplies 120VAC, but that's probably not what you have in mind.

Otherwise? I did an Amazon search for "12v 2a battery" and actually found things. Here's one. Would you by a weird product from a no-name vendor if it had specs that sounds in the ballpark of your needs? Hypothetically you might only need to wire up a cable to suit.