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Hi, not sure if you will see this since it has been so long, but I am doing the same thing.
I got this one Amazon 10,000 mAh
It is large, but gives 4-6 hours, if not more. You can get some smaller ones size wise and still be at 10,000mAh, but this one also had 9v output. I wanted a larger screen for my portapi, I wanted full 5 inches instead of 4 or 4.3.
Also one item which was annoying, but not end of work for the battery is there is no shutoff, once turned on if there is any drain it won’t shut off. Not a huge problem, I just wired in a switch, but that adds more wires and work, but easily doable.

Hope that helps