Hi Paul,
I don't think that across the grain is the most necessary, as the strain comes on the kayak in the length. You sit in the middle, the gear goes all the way to the ends and the ends provide much of the buoyancy for the weight. The bow goes up when you meet a wave and the strain comes in the length. The same is true with a following wave. Much of the strain focuses on the weakest part of the hull, where the cockpit cutout is. You can lay the cloth diagonally, and if using two layers cross the first diagonally, but that is all. Across the hull is fairly useless.
JohnK