I love the theremin and encourage you to follow through with your project. You might want to consider a rechargeable inverter-type power supply, performs even better with a solar panel. It may be a bit spendy but you'll have all the juice you'll need to run your system all week and recharge other batteries etc... Check out Solarsense.com for solar panels and inverter power supplies, and you'll see what I mean. Also, plug your theremin through a delay pedal or other fx box. You can get all Jimi Hendrix on that bad boy and rock the playa, theremin style.
Good, good, good, good vibarations.
Too much is never enough.