Bean, if you haven't jumped on those 9.6v RC batteries yet, you might want to make your way to ebay. Do a search on "portable dvd battery" or "battery (memorex,esa)". There are a number of vendors selling open package lithium ion batteries that are much like the famed Walmart batteries, but are 7.2v 5400ma-hr. (Branded as either memorex or esa.) If you look around, you should be able to get them for $30 shipped or less. These are nice and small and light, and come with integrated battery charging technology, all you need to do to charge them is to plug them in with the supplied AC wallwart.
I've gotten two of these from two different ebay sellers. They work great with the V3 and should last for 5-6 hours each. I haven't run them that long, but I've run them for about 4 1/2 hours with no problems. At 5400mA-hrs, these have a lot more capacity than the RC batteries you're looking at and cost about the same. Definitely a power option to look into.