Eagle
I think those stainless ones that PF Flyer is talking about are too narrow for the '78 cutouts. The '78 is much different from the earlier vents.
I also toyed with the same notion and failed to find ones that would work. Without some hull modifcations you could make a backing plate that would reduce the cutout to accomodate a flush type vent either plastic or stainless.
I did the above and then painted them to match the hull. The originals stuck out too far and past the rub rail and were always getting cracked and scraped. Maybe this will give you some ideas
