Mark,

I used hatch stiffeners on the lids of my Shearwater Baidarka, the shape has stayed the same and matches the deck well. My paddling buddy who built a Ches. 17L with the CLC curved deck also had a problem with his hatch lids springing out to a different shape. What we did was to fabricate hatch stiffeners similar to the ones below but without the holes(which are to reduce weight), we then cut verticle kerfs in the stiffener, so the stiffener looked like a hair comb for lack of a better descriptive term, but this gave the hatch stiffener some flex so it would allow the hatch to flex to match the shape of the deck. The stiffeners were attached to the lids with the kerfs opening downward(away from the hatch). After some trial fitting and clamping with ratchet straps around the kayak and hatches to ensure we could hold the hatches in their proper shape, we removed the hatches, filled the kerfs with thickened epoxy, clamped the hatches in place and let the epoxy cure. After the epoxy had cured the hatches were the same shape as the deck and just required a little cleanup to make them look pretty on the underside.
This is actually a long winded explaination of a simple procedure. If you don't follow me let me know and I'll try again.

The pic below is from Eric Schade's Shearwaterboats site these are the stiffeners that I used.

Happy building hope this helps!

Ken