Claude that was one thing that had me concerned when I started planning this whole mess a year ago.
There still is about 8" (guessing) or so of 1/2" plywood above the opening. The remaining bulkhead forms an upside down "U" shape. Sort of a squared off mini Roman Arch. When I was tromping around the cockpit on Sunday I didn't notice any significant flexing. And this is with 330 pounds of me movinig around.
But, I don't think that I'd let it unsupported for the long term. Any flexing that we might not even notice would over time fracture the fiberglass. Or for no other reason the weight it would need to support if the cockpit were flooded (8 lbs/gal) for some reason while I'm away from the boat.
My current thought is to have a stringer running across the bottom of the opening, maybe 1/2" x 2" or 3" high, resting on the structural stringer. It would hang on the fuel tank side of the bulkhead using some wooden dowels for support. With some sort of cabinet latch on either end so it could easily be removed. Then at the midpoint a vertical support. That would attach with a pair of dowels into pockets in the horiztonal wooden stringer. The top of that support would be same as the ends of the horizontal stringer. Making it all removable gives me easier access later on should I need to remove the fuel tank or move something new into that area.
And as Ken reminded me, some cleats, maybe 1/2" overlap, along the outer vertical edges of the the opening for the covers to rest on.
The covers themselves would have an upside down "L" bracket on their back near the lower edge that fits over the horizontal wooden stringer. Fastening at the top with whatever cabinet hardware strikes my fancy at Lowes.
When the covers are in place, along with the horizontal and vertical stringers, I would think that it should be nearly as strong as the single piece of plywood was.
Sorry, but I don't have my Visio drawings here at work to post. I hope that made some sense.