Ideas/pictures of waterproof storage built into casting platform extensions

I have a ton of different ideas, but in searching on here, Tiny Boat, Tin boat and other places either see extravagant aluminum (read expensive) lids or things that don't have any water management.  I'd like to build a box to put 3700's in and place it in my extension.  Going to shorten the extension too as it is larger than necessary and takes up too much passenger room.

The light gray is the extension.  Anyone have any pictures of someone who has done this or great ideas to keep the box as waterproof as the rest of the boat?

That section is 4' wide.  Ought to be able to house a few planos.