For anyone that comes across this thread 8 years later, I tried various things and finally found the Mission Engineering 529P power bank - https://missionengineering.com/shop-2/products/power/usb-battery/. Comes with everything you need to power an HD500X on battery (polarity reverser, 2.1mm ->2.5mm adapter). Apparently it also works with the HX stomp and other Line 6 pedals that accept DC power. Pretty excited about this so I can play my Variax through my pod into my JBL battery powered speaker (along with my VoiceLive 3 for looping and subtle vocal effects) on the street. This power bank should be good for at least 2 hours according to the ratings of both devices. Probably substantially more, depending on your patch setup.