I had the same problem when I played bass in pit one year. I've seen other bands use a type of battery/inverter combo similar to what has been posted, but when I looked up the product it was quite expensive so I passed.
We got by using a long extension cord, but my permanent solution was to move to playing timpani...
In my oppinion it is an insult to the bass to bring it down to the level of marching band.
