I have been working on finding a better way to power my mobile retro pie for a while now. The only requirements were that it had enough amperage to run it, was easy to recharge, had a small footprint and a power switch. As simple as those requirement were, it was rather difficult to find one out of the box. I had even tried using a lithium ion battery pack paired with an Adafruit power boost 1000c (https://www.adafruit.com/product/2465). This answered the power and charging requirements but I had to custom build a simple power switch for it. The real problem I had with it was that I needed to build/find a case for it and that just made the footprint too big. Turns out the simplest solution is actually the best. I took a small $10 power pack battery that had a small profile and just added the necessary switch and adapters to it. Also very easy to mount given there is a giant flat space on the back of the retro pie.