Okay, wow, that's a big hatch! Thanks for posting the photo, Sea Wolf.
I was going to reply just to clarify what I said, as upon re-reading I see it was the victim of some quick editing (I needed an extra set of hands for a project I'm working on, and they materialized just as I was editing so I rushed it). Anyway, I was visualizing the more typical-sized hatch, and was just saying I'd consider replacing it with ply.
Now that I see how large the hatch is.... well, that's great for access but does look pretty unwieldy on a daily basis. Here is a thought, although mind you I'm only looking at the photo and thinking of ideas.
Use fasteners to "permanently" attach that large hatch to the fiberglass lips it rests on. Then, cut out a useful/handy size hatch hole somewhere in the middle of it, put cleats inside as usual to hold it up from below (the new hatch), and use the new, "normal sized" hatch for 99% of the time. Then, if for some reason you need to have the whole opening, you can still remove the huge hatch by taking out the fasteners.
(Might possibly consider replacing the Starboard with ply if it's too bendy, you want to glue the cleats on, or etc.) (It looks like one reason that large hatch might be sliding around is that Starboard can tend to bend, and it almost looks like the outside corners have "banana-ed" up a little, and so maybe that, combined with the slipperiness of Starboard, is causing it to ride up? Of course that would not be an issue if it were fastened down.)