Took awhile, but I've got one running now. Hanging up in my shop, doing a battery runtime test. Had a book carved but the rPi has such a huge footprint it won't fit. Will come up with something to store it in later. Have several of those battery packs. Considering gutting them or finding a way to manage multiple microUSB power connections attached to it.
That battery I had attached to it lasted about 2.5 hours. It's a 2000mah battery. I wonder how complicated it'd be to gut those and daisy chain them together. There's a surplus store near me that has a BUNCH of the things for cheap.
Got a solar panel that can run a Pi, too, but don't have an easy way to charge those batteries to boot yet. I'll continue working on it.