Matt, I can see there is a concern there. By the way, what capacity is your engine again..? Also what revs are you going to be cruising at..? I doubt it is going to be a planing proposition, so the vents don't have to be quite that big. FWIW, my 120hp Lehman is easily served by 4 vents which take the form of a rectangular tube arrangement about 10 x 6cm aperture, that comes up from the ER through into the saloon, but hidden in cupboards or under built in seating, and open out on the outside of the saloon walls about each corner, (more or less), and covered with vents that prevent incidental rain and splashes going down. Is it possible you could rig up something along those lines. That is, PVC type tubes or similar (rectangular or square rather than round), passing up from the ER in places where you could turn them with a right angle to vent, (or actually suck), through a covered vent in the low saloon wall..?
I'm going down to the boat tomorrow - having another attempt to repair of the fridge, with the help of a fridgy. I will take a few shots to demonstrate, but I think a guy with your skills has already picked up on what I'm trying to describe.
PS. I can understand it would break your heart a bit to cover those vents you made, as they are, as with everything else, beautifully done - works of bloody art mate - but if you saved the cut-outs, you could epoxy them back in, and no-one need ever know. Our lips are sealed...right guys..?