Do you have access to a fiberglass store? You might look for a small piece of Plascore or other honeycomb composite.  They're very easy to work with and when it's glassed on both sides, it's stiff.
On the bare hatch glass I would grind it down and if the sliding hatch is already stiff (can't twist it), then add some 3 oz CSM (chopped fiberglass) and roll on the resin (I assume poly).  If it's not already stiff then I would add a layer of 1708 CSM side out on top of the CSM added as the first layer. That will stiffen it up.  Then cut your composite (you could use plywood, 1/2 or even 3/8" with glass on both sides is beefy enough. Trowel on thickened resin on the hatch, as Fishhead mentioned, lay in the core and you can wedge a 2x4 up against it to press it into the resin. If you're going to top coat it with a layer of glass then you might want to bevel the edge of whatever core you decide to use - 1708 doesn't like to traverse corners.

Thanks Rick. I will give that a shot.

I posted a picture but have not managed to get it on the post. Working on it.

Posting pics is next.until then, thanks for the input. I was going to finish more commercial grade so liner is out. I just wanted to be sure it wasn't going tol fold like a cheap suit. Looked at your build. Very helpful and will probably add a layer of 1708 as suggested or at least some ribs.  Thanks

Flathead screws are indeed the absolute worst! There are few different ways to rebuild these hulls to make them last as long as they did. What you have to ask yourself first is what type of finish do you want and what your intent is with the rebuild. If your looking for a nice finish inside, the liner will make it much easier not to have to fair/finish the inside of the hull after you tie the deck into it. However, when you cut the deck out and try to leave the liner, you'll want to tie the deck to the hullside to keep some structure. This will leave you with a liner that has to be re tabbed into the deck  so it has to be trimmed (assuming the deck is thicker or at least will be raised). The last option that I don't believe you see often is removing the liner as one piece (hoping it doesn't crumble)..and recoring it filliped upside down with divinycel, coosa, or other composite materials Check out my flatback thread and you will see what I mean. There are a LOT of rebuild threads on here to look over and many proper rebuilds to follow. Share some pics and it will really help with guiding in the process and perspective to answer any questions you may have.