This is done all the time for amateur radio emergency work. Usual deal
is to get one of the marine battery boxes, or something that will fit,
put in a couple of 12 cigar lighter sockets, maybe a switch, fuses,
and pilot light, and you are good to go.
Mine is a "sportsman's dry box" from the local sporting goods store,
sockets from Radio Shack, and charger, etc., from local hardware
store. I also have a microphone clip on the end so that I can set my
radio on top, and keep the microphone from flopping about. For smaller
endeavors, get one of the SLA gel cells like are used for alarm
systems or emergency lighting systems. Depends on what current draw
As for USB stuff, there are USB adapters that plug into a car cigar
lighter. I have enough space in my box for the charger to live, and a
couple of odds and ends of cables, adapters, etc. I also have one of
those plug-in volt meters to check the state of charge of the battery.