I was happy to find this box from Plano, because it keeps things safe and dry at a very low price. It'd be perfect for storing batteries or a small-ish phone. 6 1/4" by 3 1/4" are the internal dimensions I measured. Right now I'm using it to store all my stickers.

With the gaskets and clamps you can tell it's built right. I took off the top/bottom clamps since I find them unnecessary, but you'd probably want them if this thing is going to be far underwater. Definitely made of softer plastic than the guide series boxes. Will buy more when they come back in stock.