I've done that with a CLC cambered deck. What I did was make a sort of wooden bowl by stacking rings of 1/2 inch plywood on top of each other. Larger rings at the top, smaller at the bottom. The bottom of the bowl was a piece of 1/4 inch ply. I epoxied the rim of the VCP hatch to the bottom of the bowl and cut an opening in the bottom. The top of the bowl I shaped, by trial and error, to fit the underside of the deck. Then I glued the whole thing in place. At some point, once I had the shapes about right, I sanded away the stair steps created by the 1/2 ply on the inside of the bowl.

Is that clear as mud?