1 3/16" O ring is more durable and stronger than rubber bands and costs 50 cents at the hardware store.
Oh, and when you swing by a Radio Shack store, pick up a spare. The light is great but those battery holders stink; if you run them in the rain and don't immediately dry them out, they will rust and become unusable. Luckily the holders are like $2 at RS.
__________________
Work: the 8 hours that separates bike rides.