Vaguely related i.e. involves racing on water.
In my spare time I race marathon open canoe; two person, sit 'n' switch, American style / sourced Wenonah C2.
When there is a bit of side wind the canoe really gets pushed as it has two sides to the wind, inside and outside.
For the 125 mile race we made tonneau covers out of tent material velcroed in place but by design open where we sit as we also faced 77 portages. Made a huge difference to the boats tracking in cross winds but for reference we were only travelling at 5-8 MPH for a shade under 24 hours.
On the other hand marathon kayaks have a large cockpit opening (as the knees sit high and the legs also pump / drive the boat) with a lip and a spray skirt that is elasticated to attach to the boat and around the waist. White water boats have a smaller opening and really taut neoprene spray decks. With that in mind:
What about a solid tonneau (i.e. thin carbon fibre / kevlar or similar really well attached to the pontoons; sikaflex adhesive?) with cockpit opening lips built in and a spray skirt or at least deflector shield. At 70 plus mph I would imagine a full fabric one (even with reinforcing) will simply flap and potentially be worse than nothing, or tear off and I am not sure I would want a sheet in my face at 70 on water