I think battery boxes are overkill. Batteries are sealed so the water is not going to bother them. Here is how I do it. Wires soldered to the terminals, a coat of red liquid tape and when it dries, a layer of marine goop on the terminals. Now the battery is completely waterproof. Add a waterproof quick disconnect plug. The "battery box" is an insulated lunch bag. I strap the bag below the hull to the mast tube. Super simple and super cost effective.
