The ribbon like material sounds like it could work. It attaches to a rope as it finishes its journey across the tramp.
That sounds like what I am after but the next step is how to pack the top and bottom of the casing so the board can move freely up and down but with as little sideways play as possible. It would have 160mm of ribbon exiting the hull so mounted on the sides it might catch the water flow. On the leading edge it would flow nicley to each side. A roller made from a pully mounted at bottom of the caseing pushing against the leading edge would help hold the ribbon in place. Still I am keen to hear more veiws on the subject as I am still turning the cogs in my brain for the best solution.