A Math Question: Buoyency

Here's an interesting question that I hope someone can help me with. I'm looking to make a floating light system, but the only batteries I have access to are FIRST batteries. They're a bit big.

So here's exactly what I'm looking for, and I'm hoping to learn some math along the way. I have exactly 15 pounds of electronics and batteries, and I need to know the volume of the container that I'd need in order to make this float. I'd rather do the math for this first, so I can build the container out of Lexan, rather than doing the guess and check type experiment. It would be floating in standard pool water (chlorinated fresh water), with not too much water disturbance.

So how might one go about finding this out?