Found a possible juice pack for ya'll, the Qidian QD-184-SX
its 2 usb + 5-13v 3A dc jack, it uses standard (read replaceable) 18650 cell batteries
its sold with or without cells, single unit price is $17.35 (16 at 5 or more) without cells and goes up from there depending on how much battery you want.
I've been using them for field testing cable modems, seem to work without issue.
(edit)
Sorry I should have been more clear, I've not used the 4 cell version, i've got the 6 cell version (20.4Ah baby) of the same brand, but from the pics it looks almost the same, just smaller ..
$8.95 on the hak5 store, don't know about you, but I try to support them when I can. but yeah if you have a Rpi you can use that, you just need to, disable the console being spawned on the port apt-get install minicom remember rx to tx and tx to rx as you cross devices ...