Just for some more ideas we use one of these (or its smaller cousin)
apc universal laptop battery
this one too
Just do an online search for "universal laptop battery" and you will find many devices. The advantage of these is they're typically small enough to either fit under the netbook or right next to it. All day Saturday and part of Friday we ran one of these, kept it plugged right on sitting right next to the classmate. Once we got back to the pit we plugged the classmate charger into the battery.