There was a pretty big thread about this in /r/Android the other day. Is an external battery pack out of the question, or are you just looking for a more integrated look?
For half the price of the Mophie you can get a external battery back that will charge your device several times.
Anker® 2nd Gen 12000mAh $49.99
I considered the Mophie as well when I was looking for a little extra boost through the day, and found that the battery pack fits nicely into the pocket, and allows me to keep the choice of case, or lack there of. I own the 12000mAh one that I linked to, and it allows me to charge anything USB powered, which is fantastic, as it won't be a dead weight trying to sell it when the time comes to move on to another device.
I use this. It's great. Extremely useful whenever I'm traveling or otherwise away from charging for a while.
It's not the cheapest, but its very high quality. It can charge itself and your phone at the same time (in case you're somewhere with questionable power quality), it can deliver high current to tablets (including 4A to an iPad), it can charge multiple devices at once. It's just great. Strongly recommend.
I camped last year with an Anker 12000 mah battery (50 on Amazon) and it gave me and my gf 2 full charges apiece on our phones. My friends with Samsung phones did a battery swap via a promotion they had all weekend.
I see a lot of critics regarding phones, and that's a nice opinion to have. Most of us want to take pictures, capture videos of random shit, play music at campsites, and meet up with our friends.
TL;DR: Get a nice external battery if you camp, don't go with solar, use the charge stations as a last resort.