My advice would still be to go talk to a boat-builder...I'm putting in a new floor in the cockpit so now is the time to do this modification
I don't think adding that aluminum hatch would compromise the deck, if anything it would make it stiffer. The perimeter ring would be stronger than the original glass that was there. It would be no different than adding a door frame in a house.(I know boats are different, but it's the same structural concept)My advice would still be to go talk to a boat-builder...
You'll need someone to work out a design for the new floor that incorporates all the structural stiffening needed to support the floor around the hatch and work up an appropriately sized and shaped access hatch at the same time. Having done this on my boat already I can tell you it's a non-trivial exercise and there will most likely be much discussion and compromise.
That's how my Kaufman 47 is.. Whole shebang comes out. It doesn't have the latches like you but I will be adding them, I like the way yours is. Undo them from the inside in the unlikely event going into the lazerett or the access hatches on the inside don't suffice.If going the way of making your own, check out Albin Vega's, which have a removable cockpit floor that is very simple and could easily be copied. Basically the entire floor is the hatch that screws down to a fibreglass ledge. the cockpit floor is fibreglass with polyethylene strips glassed to the underside to provide rigidity. If the "frame" of the cockpit floor left after cut seems too weak, this could be easily fixed with aluminium angle reinforcement around the perimeter.
I used to own a Vega and found some picks I took of the cockpit floor if this helps the think tank:
The floor/cover undergoing reconstruction with full ply reinforcement for strength and sound proofing:
cant put on in the rear quarter berth the exhaust is in the way and i would loose structure from the seating and where you entre and exit the boat by putting one thereCan't you put in an access panel in the quarter birth? Seems a lot easier than going to the trouble of making a hatch. I have seen some nice hatches over engines, like the one on the Lancer 36, but seems to be a fair amount of risk that it could come open in a knock down.