Best portable way to power the board?

Best? I am not sure...

A couple of ways that I have hooked up boards like this to a Robot that I typically use 3s Lipo batteries for the servos is to either use a simple Battery Eliminator Circuit(BEC) like: https://www.amazon.com/Castle-Creations-Bec-Switching-Regulator/dp/B000MXAR12/ref=sr_1_1?ie=UTF8&qid=1477147243&sr=8-1&keywords=BEC

With these I hook them up to Lipo battery (through switch), and get the correct Barrel Plug to plug it into the power connector.

Or, if I am building my own board to interface to the Host board such as the UP, I put on a Voltage Regulator or DC to DC converter that can supply enough current to feed the UP board. Current board I am playing with uses one of these: http://www.digikey.com/product-search/en?keywords=811-2179-ND with the proper trim resistor to give me 5V.

Again can not say this is the best, but...