My plan for seat hatchs.  Below is a picture of the hatch-cover top-view, the seat without the hatch-cover top-view and the hatch cover and seat front-view.
I decided to put seat hatches on the top rather than the sides.  A method to keep water out is desirable.  The design above shows a hatch with a channel around it to divert water from going into the hole.  The seat is made from one piece of 3/8" ply as the original plan. The hatch cover is made from the cut out piece from the seat.  The hatch cover is designed to be hinged in the rear.  When the hatch cover is down it should act as a box lid and cover the built up lip going around the hole.

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