If price is your main concern make one yourself. Specially if you don't need 110
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buy a 12v battery of your choice (deep cycle are best, but car batteries will do in a pinch, UPS batteries if you want small)
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buy a tool box of adequate size for your battery, allow for extra room
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order a "float charger"
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Order USB and cigarette lighter ports as desired
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wire it all up inside the toolbox
I've made a few with minimal effort, they're great for camping and my latest one even allows me to jump start a car
There are plenty of guides out there with more detailed steps
https://www.youtube.com/watch?v=49Do__99Yr4
I'm currently making one for a friend, let me know if you guys want to see pics of the components