A closed freshwater cooling system for a steel hull is pretty simple. It can be essentially just two heavy channel or half-pipe sections, welded to the side of the keel. A wood or glass hull will of course require something bolted to the hull. There's no need to do that with a steel boat.
This idea was shown to me by Clyde Flowers, founder of Custom Steel Boats. He built one of the first Diesel Ducks (Wendell's boat) and this is how Mr. Flowers designed the keel cooling. I liked it so much I borrowed his design! As I keep saying, NO idea is hatched in a vacuum. EVERYTHING about boats is a product of collective ideas that over the years have proven to work. This is a good example of that! Ask your engine dealer how much water is needed for cooling and make the channel accordingly. No reason to be exact, either. I'd go well oversize so there is capacity for an added gen plant if you feel like it.