Once I had a 'mold' which imitated the shape of the deck underside, I could proceed with building the hatch recess.
I used a foam recess blank (cut on the bandsaw with a quick-n-dirty circle cutting jig) and shaped it to fit the mold underside. (These steps were identical to building a hatch recess on a new boat, working on the underside of the deck.)

Once I had the recess piece laminated and trimmed, I slid it under the deck. Note- no bulkhead was in place at this point!
Also, I rough cut and undersized hole where the recess would eventually be formed, so I could work through the deck.

It pays to take some effort to get a clean fresh bonding surface on the under-deck- solvent clean +a good amount of sanding is a good idea, IMO.

To 'clamp' the recess in position while bonding, I used (a lot of) self tap screws via holes drilled through the deck to 'pull up' the recess into position. The screws were easily removed once the recess was bonded, and the holes were filled with epoxy and milled glass fibers, with wax paper under the deck to smooth out the epoxy and reduce sanding later.

After that, it was simply a matter of grinding away the deck until the recess was revealed completely, and filling, fairing and sanding the result.

The entire deck was primed and painted with Interlux 2-part primer and Perfection 2-part paint.

Then the paint was scraped away where the hatch rim was to be bonded, and the rim was bonded with epoxy tinted with black iron oxide and thickened with silica. Previously, I had drilled and countersunk 8 holes through the hatch rim- epoxy 'squeezed up' through the holes and formed a 'mechanical fastener' of sorts. (Depending 100% on the Chemistry Set when bonding plastic makes me nervous. BTW, on a previous boat I had used 3M5200 for hatch rim bonding- fortunately I was at the local beach when the 5200 bond failed.)

I also added some (ugly but very functional) spare paddle holders to the front deck.

The rear deck got a day hatch as well as a larger hatch.

I added a bilge pump and thru-deck at the same time....hiding everything with that fresh coat of paint!