Right, on the slack I sail it about that slack as well and have won both the East Coast Championships and the North Coast Championships and am presently leading the Wave National Series Championships. All with a loose rig.

On the hatch, I believe they are flat. Just use a lot of sealant if the curve is too much and don't tighten down the screws too much or the hatch will be hard to get in and out.
Never tried the curved.
I believe we have 5" covers.
Rick