I’m going to go against the grain and recommend the Jackery line of battery packs.
Well, this one was a Gold Box deal last month, so I picked it up. Seems reliable so far, the case is really rugged (and waterproof, they say, though I haven’t tested that yet), and it has a flashlight and compass as well. With a capacity of 9000 mAh, you should be able to charge three phones or so before you need to refill.