My brother-in-law gave one of those plastic green ammo boxes several years ago with some stuff inside for an Xmas gift. I put some high density foam in it and works perfectly to store my battery. Made some room for a hand warmer or two as well. But, to be effective, I'd have to drill a hole in it so the hand warmer can breath. But, I primarily fish out of a wheel house, so it's not an issue. Just sharing, so people know hand warmers quit working in air tight containers, like a cooler. Also, the ammo box is the perfect size and would easily hold two.