I agree with Bill, Thats some really nice work. I'm not quite sure how you got to the point of the recess and flat surface based on the photo of the deck shape mold. I have done it in wood but I'm always interested in ways of doing that. Either way - beautiful job. Too bad the photos didn't make it because a lot of people would be interested.

Thanks Bill, Jay- I appreciate the kind thoughts.

Once you've made the deck shape mold (assuming your deck has near-constant & uniform thickness) then you can treat the underside of the deck mold just like a section of the underside of the deck.
From there on, it's pretty much a matter of following one of the 'standard methods' for making hatch recesses, like:
http://www.blueheronkayaks.com/kayak/whiptail/hatches.htm

I may be doing a front hatch job on another Express (the yellow one in the pics) - I'm waiting on word from a potential buyer - and I'll take more pictures (and SAVE 'em in multiple locations....).