I put VCP hatches on my Patuxent 17.5. It's a bit difficult to describe how I recessed them, but I'll give it a try.

First I cut a flat ring of plywood for the hatch rim to rest on. It is about an inch larger than the O.D. of the hatch rim. Then I built a hollow cone of stacked plywood rings. The narrow end of the cone fits on the flat plywood ring, around the hatch rim. The wide end fits against the deck. I used a shurform tool to even out the stair steps from stacking the plywood rings and to contour the top of the cone to fit the underside of the deck. To contour the top of the cone I used the same guide, cut to the radius of the deck beam, that I used to bevel the sheer clamps. This worked well for the round front hatch.

I tried to do the same thing for the rear hatch, but the fit of the oval ring and the less cambered deck made it a more difficult process with long, thin spacers running down each side. I wound up just gluing the flat plywood oval to the underside of the deck where it naturally made contact, then filling the gaps with epoxy putty. With a drum sander on the end of an electric drill I was able to smooth everything out.

In both cases the VCP hatch rim is glued onto the flat playwood ring with thickened epoxy.

I'm pleased with the way the hatches look and they are WATERTIGHT.