Check out this Forum discussion about rechargeable batteries for the Raspberry Pi:
The Powerboost 1000 is probably a good fit for this project. I can’t really speak to the space available in your enclosure. I think the limiting factor would be the size battery you choose to use. If you want to keep your install clean, you can run USB power from the Powerboost to the Raspberry Pi, and use a Panel Mount Micro USB from the Powerboost to the outside of the enclosure.
