Just forget about carbon fibre. It looks pretty, but it conducts electricity and is opaque to radio. Use fiberglass instead.
Again, fiberglass. Completely forget carbon fiber exists.
I’d recommend you make the enclosure so that instead of bolting to the deck along its perimeter, just hang it from two bolts on the front and two bolts on the back. Put butyl tape (bubble gum consistency) all around the top edge, inside the bolt holes and lay a sheet of flat rubber on top. Put a cutting board on top of the rubber with some weight on top for an hour to squeeze the butyl tape, then bolt to the deck. This way, you have completely sealed from water ingress. It doesn’t even matter if the top of the enclosure is flat and the deck is concave, the tiny gap makes no difference this way.
So you have like a “tub” hanging off the bottom of the shortboard whose only hole is a charge port, and you’ve sealed the backside of that. No other holes, you should be able to toss that in a lake if you want to.