Rob Macks at laughingloon.com describes a fix for strip planked hatches with a similar issue.
He describes removing the glass on the inside of the hatch to remove the stiffness. You would have to remove the stiffeners going across the hatch for the tie downs.
Then he puts packing tape on the hatch recess as a separator film.
He shims the centerline of the recess so that if you force the hatch down in to the recess over the shims, you are "overbending" the cover.
When you have this set up, you take off the hatch, glass the inside, and re-clamp the hatch down.
Allow it to set.
When you remove it, take off the shims and packing tape, the hatch will touch down first on the edges.
You would then have to replace your tie down stiffeners.
His web site has suggestions. You also can buy his building manual ($25 for a great deal of information) with the same information.

Given the stiffeness your tie down adds, you might be able to just remove the cross stiffners, taper them more in the outboard directions, and reinstall clamping the cover to the stiffeners, forcing more curve.