Another possibility that you might consider is plugging in an external 12V battery. The unit has a coaxial DC power jack that it uses for charging so if it will run while charging, that would work and the internal batteries would be charged from the 12V battery if needed. The internal batteries may not charge completely when the 12V battery is run down a bit.
Something like this would give you at least 4 times the run time.
http://www.frys.com/product/6607874?site=sr:SEARCH:MAIN_RSLT_PG
Of course you would need a conector and a small charger for it. A battery specialty store like Batteries Plus could set it up for you or you could do it yourself.