From my website:

fter building the wooden recesses for my kayak, I decided to change plans and go with a fully glass recess. Building these requires much less work. First cut the holes in the deck for your recess, round the cuts with a file and sandpaper. Dye the exposed surface.
fittings.jpg, 74 kB

After the holes are cut, a mold must be built for each recess. This is easily accomplished using chunks of green foam that is used for floral arrangements. Cut a block of foam that is about 1/2" larger than the hole for the recess, and about 1.5" deep. From the outside of the deck, push the foam through the hole you cut. Keep pushing until the foam has come through the opening 1/4" deep. You'll find that pushing the foam through the hole is extremely easy and it perfectly matches the required shape.

When the foam has been pushed 1/4" deep, tape it in place, and use your fingers to round the corners of the foam. Then use your gloved finger to apply some PVA mold release to the foam. It's best to use, at least, two coatings, because the foam may absorb the first and second layers completely. As soon as you have a coated surface, the fittings are ready for glassing.

Mix up a batch of thicken epoxy. A mix of fumed silica and mircoballons will ensure a light and no sag mixture. Gently spread the mixture over the PVA coated foam making sure not to tear the PVA. The key point here is to spread enough of the mixture to allow the fiberglass that you'll lay up next to easily curve from the mostly flat deck around the curved fitting. The joint between the deck and the foam is the most important location to think about. You can have this mixture be a final outside surface, but it's best to use purple micro balloons, which will blend in with the blood red dye or use black epoxy pigment to make the mixture black.

Next cut out 6 4" by 4" patches of six ounce glass for each deck fitting. Wet these out on a plastic coated surface and then lay them over the foam mold trying to avoid air bubbles and moving the thicken mixture around a bit to arrive at a smooth surface. Cutting all the patches at ounce is easiest and allows the thicken epoxy to set-up slightly, which makes it easier to work with.

Once cured, dig as much of the foam out of the recess as you can. Then use cold water from a hose to remove the rest of the foam and the PVA. Acetone can also dissolve the foam, but make sure your epoxy is completely cured before trying this, because acetone can weaken uncured epoxy.

Next is to mask using masking tape around each fitting, make another batch of thicken epoxy and fill in any imperfections that you may encounter. Then lay a layer of wet 6 ounce carbon fiber into the fitting from the outside of the hull. Once the carbon fiber cloth has had hardened enough to cut with a razor. Cut into it removing any excess that rises over the masking tape and above the deck.

Drill a hole, attach a Valley fitting, and use thicken epoxy to cover the nut on the inside of the hull.

This method would also work using just a stainless rod instead of a Valley fitting. For this layup, you'd have to either settle on the finish provided by the thickened epoxy, or lay a layer of carbon fiber directly against the foam mold and make sure no bubbles are formed. For this method, after PVA has been applied push the stainless rod through the foam, making sure it is directly against the deck. Then lay the glass and thickened epoxy over the foam and steel rod. To make sure the rod doesn't spin, bend the end of one side 90 degrees before pushing it through the foam.

Notes: Jay Babina suggested using ping pong balls cut in half for round recesses. The balls stay in and are painted black after glassed and gooped into place. Brian Nystorm writes, "Molding deck fitting in ice-cube trays. I've seen them in various cavity shapes and some trays are polyethylene, which epoxy won't bond to. It would seem like an easy way to easily make consistent deck fittings."