Whatever size you choice, I recommend building a pair of "trusses" for each hole you cut into your hulls.
I repurpose some old battin stock for this.
Start by taking a narrow piece of cardboard and cut out a contouring shape that matches the underside of the deck (internally). As if the cardboard is perpendicular to that surface, hanging tall for strength.
Once your happy with this cutout, transfer layout to a battin that spams longways almost touching each hull. Cut that battin to fit under the deck about 2" away from the hole you cut into the deck. This will leave you with room to fasten the access ports to deck and not interfere with the trusses. Place one on each side to reinforce the area you just cutout.
These will become your trusses.
Now fiberglass these trusses in place by bonding with fillets to serve as a structural brace. Keep foot print somewhat small on the access port side.
If you flip your boat over and place on top of some sawhorses, you can use gravity as a friend to help keep the wet resin from dripping.
This method with keep the deck solid at your new access ports. But still, don't step there.