Ron,
Let me give you an alternative way of looking at things. do not cut holes in your new boat for hatches. they are unnecessary. In the end pour in each end of the boat, put one end of a 12" cord. Tie in a little pulley into each end. Put a long, circular rope in each end, and loop them together at the middle of the boat.
Bend on a 3' cord onto the long ropes. The first piece into the fore and aft ends should be something small that will fit well up into the stems. A folded air mattress, small dry bag, etc. Pull the rope to run it up into the far ends. Push in the rest of the load up under the deck.
This simple system alleviates a need for bulkheads & hatches. You can use a cockpit liner if you like to keep excess water out of the cockpit area.
Think this one over & ask questions if you like.