The deck coring looks good, be sure you seal it with two-part epoxy to hopefully keep it that way. I would cutout about 1" of balsa all around the opening, paint the balsa with epoxy and then fill the void with filled epoxy. I would probably use chopped fiberglass filler as well as cabo-sil in order to strengthen things, and tie into your hatch coaming.
Hope to see more pictures as your project progresses.
I asked because I cut into part of the copper mesh and strap and wondering if I needed to rebond the system . The main salon needed more light,I cant fit the sofa , gen fridge, etc without alot of difficulty ,so I designed a removable hatch/ sunroof in the center of salon.