I came across a portable battery bank project. It is housed in an ice chest. Would probably work for a hitch basket mount for boon docking situations where you need some extra power but don't have built in compartment space. A McGyver solution to be sure.
I will leave it to the electrical experts in the group to evaluate as to its suitability. I see one issue with it, the 12v sockets he used should have been the type made for outdoor installation on trailer hitch or boat applications and installed with RTV silicone to prevent water infiltration into the box. Any vent openings should have louvered covers to prevent water infiltration.
http://www.alpensteel.com/article/53-101-energi-terbarukan--renewable-energy/2959--the-standard-battery-bank-on-the-airstream.html