Thanks for the tip, I didn’t know such an adapter existed :)
BTW do you know if a 5V/3A battery pack exists? Everything I find is either only 2A or when they say “3A” they means two USB ports, one with 2A and another with additionally 1A.
If you want to go big (which brings up a discussion about what is portable) you could build your own power supply with any size / voltage of battery set by adding a DC-DC LM2596 DC-DC converter. It has an operating input range from 6V to 40V. There are many sellers on ebay of this unit for a wide variety of prices but starting in the sub $2 range.
I have used several of them to power a Pi system including a mobile broadband router, Ethernet switch and Pi from the batteries in my solar powered home power system. Although not portable it does prove that these units will work with various power sources to provide 5Vdc output.