I am not really reffering to the hatch, I mean't the HATCH COVER

On the Shearwater Double, the hatch cover is two layers of ply.  With mine, I could split them with chisel, then re-expoy using weights to bend the hatch cover.

Or, add a second layer, making sure it is smaller and stays away from the hatch lip (about an inch in all around).  And bend using weights after you glue them together.

And my middle hatch isn't a great fit, but I'll live with it.

Good luck!

It is common for the hatches to relieve themselves when cut out from the deck.  You can make an athwartship stiffener (or two) the same radius as the deck and epoxy those to the underside of the hatch.  That should hold the hatch curve to the deck shape.

I had a similar problem.  Someone suggested heathing the hatch with a heat gun or heat lamp and once the epoxy softens you can bend even a 2 layer laminate.  You may want to make up a form with a bit  more than the correct curve, warm up the hatch and then place it over the form and clamp the edges down.  You will need to use plastic tape or wrap to keep the warmed epoxy from sticking to the form and may have to do some sanding or filling of the epoxy to resmooth the surface after clamping. 

 If I recall, did not need to straighten mine a lot but it was sort of off on the diagonal.  I clamped it over a piece of 3/4 inch plywood oriented vertically, that I had scribed and cut to match the curve of the deck.A couple of quick clamps with 1x3 plywood pads covered with tape spread the force.

 It worked.  Hatch is good and watertight.  If at first you don't succed, you can always try again slightly differently.

 Good luck.   Ed