What I really find annoying is that on all the commercial solar USB chargers I see their internal battery is only 1000 ma. That isn't a lot. A rechargeable AA battery has between 2000 - 3000 ma of current in it. Once again, we can do better.
We need to use rechargeable batteries for this project. I prefer NiMh AAs over everything else because they're easy to find, cheap, and reliable. You probably even have a few at home. Since we're using two AAs in this project our charger will have 2000 - 3000 ma of current. You could even have two sets of AAs in parallel and boost that capacity to 4000 - 6000 ma.
An added bonus from rechargeable AAs is that you can take them out of the Solar Charger, charge them up or replace them, and be on your way.
If you're making this project into a very small container you can use a smaller NiMh battery pack. You can find these at places like American Science and Surplus as well as many places on the web.