yep, that's a Cape Charles 18. The deck looks kinda dark to be Okoume, I wonder if it's 3mm Sapele.
The filleting business with the hatch ribs isn't that applicable here. Simply cut two strips of thin ply or any thin bendable wood and put them sheer to sheer. It's at the aft hatch where it's critical as that's where one climbs back in to the cockpit for rescues. Just find some clamps with long arms to clamp against the strip between hatch and sheer.

Ironically the old Cape Charles box type hatch covers were pretty strong for how light they were even though the decks were fundamentally weak for any use a "sea kayak" was designed for. When the Chesapeakes were developed the new hatches were actually less durable because the curve of the hatch ended at three 6mm ribs and the rib would pull away after successive flexing where the hatch straps pulled the hatch over the rib and during rescues any weight pushed the edge of the hatch off the rib. Coupled with unglassed decks,,,you got a recipe for aft deck cracking and failures when 200lb paddlers used the kayak as you would any hard/composite sea kayak. Which is why the present Chesapeakes have glassed decks and glass tape under the ribs extending past the ribs. With your old box style hatches there's more glued area where the sides of the hatches are stressed.

When I worked at CLC there were some old Cape Charles hatches sitting around made with 3mm ply and holding up fine,,the original Chesapeake hatches made of unglassed 4mm and three 6mm ribs were being rebuilt continually as most developed cracks on the top of the hatch. I rebuilt about 18 hatches because all of the demo hatches had black line cracks above where the rib ended and some failed catastrophically in rescues.

You appear to have a perfectly servicable and restorable kayak, sure it's worth getting a coaming on and getting it paddleable. You won't have to spend more than $100 for the coaming, epoxy and aft hatch repairs.

Re. aft deck strength, just from a lazy builders perspective working through the aft hatch you could add something midway between the aft hatch and coaming on the underside of the deck to make it stronger, whether it's another set of strips or a couple strips of 3"-4" glass tape. If you hang the kayak about chest height it'll be possible to work through the hatch.