I know this is late to the party here, but I have an idea to add.

Marine battery boxes come with a tie down strap that is intended to be secured to the boat floor. The strap usually has 1 or 2 "cleats" I'll call them. Instead of mounting to a floor, use small bolts and washers and mount the strap to the sides of the box, one strap holder on each side. then you put your stuff in the box, put the lid on top and strap it on.

This will not be water tight, but definitely water resistant due to the handles molded into the bottom of the battery box and the way the lid fits over them. I would bet that with some strong adhisive and some closed cell foam a guy (or gal) could glue foam around the inside of the lid so it contacts the top of the battery box, thus making it even more water tight.

Just some thinking....