Do you actually need a waterproof case? Lots of drone and rc people are just spraying boards with conformal coatings, and the seem to do fine with occasional dunkings even when powered up in salt water. I've seen quadcopters still "working" under water. I've seen people making submersibles that use known non-waterproof 3d printed boxes, then pumping them full of marine grade grease to eliminate any air voids that would allow water ingress.