Remember the actual fiberglass is very thin, the bow will come out without too much problem.
I used one piece of 3/4' plywood for my larger hatches as backing/core. There was a slight bow like yours on my front hatch cover, however after cleaning the old crap out I turned it over and laid it flat on concrete, then stood on it, it took out the bow quite easily. Cut the plywood as described above lots of epoxy mixed with cabosil sandwiched between the plywood and roughed up glass, laid it on the driveway with the boat trailer jacked up, set the trailer tires back down centered on the plywood, cleaned up the epoxy that oozed out, came back a few hours and a few brews later.... all flat and epoxied together. Finish and seal the wood as above.
Remember.....screws are not your friends in wood coring!! If you want to go crazy, where the present screw holes are, picture #4, you will want to re-use their locations. After the wood is epoxied to the cover and before you seal the plywood, bore 1/2" to 5/8" holes in the new wood only, then fill with epoxy/cabosil and you won't have to drill through the wood, just through the new fiberglass plugs. I know it's overkill, but not hard to do.
Good luck!
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