Another battery option is one of the "thumb" sized li-ion USB batteries designed for iPods...
Like this one:
http://tinyurl.com/j6yfbI got mine for under $19 shipped.
I haven't run it all the way down yet to see how long it will last, but between it and the internal MT battery, it should be good for atleast 10 hours.
I attached a pic...hopefully it worked.
And if the tinyurl.com doesn't work, search ebay for Item # 5876777614
Product Specs:
* Type: Li-ion
* Voltage: DC 5V
* Current: 650mAh
* Dimensions: 72x18x21 mm
* Weight: 45 g
* Output Capacity:2200mAH
I like it better than the AA battery boxes because then I don't have to buy additional AA's, and it's still rechargable.