I'm going to say its a 12 hour project broken up over a few days. The first thing you need to do is flip it over and use a rotary cutter to cut around the edges to free the "bottom" fiberglass. If the hatch is bad at least one edge have already broken. Then carefully separate the bottom. Then you have to get the plywood squares out. I used a wide chisel and popped them out one at a time. This is the cleaned up hatch.
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I used Coosa board. It's a composite material this is 3X the cost of marine plywood. It won't rot. You can use plywood but you have to be careful that everything is sealed. I used West System epoxy with SLOW hardener. It was done as one piece but could be done in squares. I vacuum bagged the part to squeeze the fiberglass shell and the Coosa board together. This get's all the air out and brings the epoxy to the top. The result is a more complete bonding
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Stiffener support added.
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All the screw holes were over drilled but not thru the fiberglass hatch. Then the underside was covered with fiberglass cloth and resin and vacuum bagged again to get the air out. Afterwards the screw holes were drilled out from the fiberglass side. Sorry - I don't have a finished picture.
I recommend using epoxy and not polyester. Polyester isn't a good choice for repairs. It doesn't bond well to "old" polyester which is used during the build process because it's less expensive. Difficult to control the "working time".
Also vacuum bagging isn't required but it isn't hard to do and results in a stronger part.