Killer vessel! I'm jealous. My MK2 Futura definitely has less beam and strapping my Brewer longboard across it is like having airplane wings on the zodiac. I also wasn't comfortable with having the board resting on it's rails, I prefer to distribute the pressure over a larger surface area. I didn't want the boards catching that much wind either. At first I used cut out ethafoam "bones" to cushion them when strapped on the side tubes, but the flex of the tubes appeared to put a bit of stress on the boards. So I made two racks out of Holland Speed Rail fittings and aluminum tubing. The mounts extend all the way across the benches and bolt on to each side so they can handle the leverage of the rack arm. The pipes running across the bench make great lashing spots for other cargo too. The rack arms are easy to remove when not in use, but the bench rails stay in place. Since the aluminum floor section doesn't flex much the racks stay in one plane and the boards are not tweaked even when launching over waves. It sure looks like you have a good system for your boat and I would only comment that PVC pipe is not particularly UV resistant and will become brittle over time with sun exposure. If you intend to use the system you have for an extended period of time I would at the very least paint it or consider replacing the pvc with aluminum. Rack failure will always occur at the most inconvenient time...
the boards float above the tubes