My GoPro 3 can do 2-3 hours @ 1080p30.
The batteries range from between 1050 and 1200mAh in GoPros. So a powerbank should have at least 4-5 amps to give so you can extend the battery life by 3x minimum. AndyManCam runs a
Noco XGB6 which is both rainproof and has decent capacity, so you can mount on the helmet too if you want. Should be pretty snug in a pocket.
Either way, what matters is capacity, I can't recommend anything specific as I dunno what is available in 'murica, but look for more than 20w or 4amp capacity and you should be good. Bonus if it's kinda rugged.
Hope it helps.
