After some bad experience with the 18650 battery format, I thought it best to do a tear down on the battery pack I finally decided on. The tecknet usb power pack comes in various sizes, but the smallest 3300mAh version will run a model B pi for 6 hours. I'll probably switch to the model A in future for even longer run time.

You can read more about it here: https://github.com/mattvenn/tecknet