I decided to install a Lewmar low-profile size 20 hatch to ventilate the head compartment. The head will already have an opening portlight but the more ventilation, the better and there's no serious risk created by this hatch, so I figures why not.
I made a point of getting hatches that have a flange rather than the Lewmar hatch models with a flat base, because I figure the flange provides a bit more security but also because the trim kit for the hatches only fit on the models with the flange base. Since I definitely want the fly screen attachment, I need to get the trim kits, and so I had to order the flange-base hatch.
The installation process was pretty straightforward: make an outline of the hatch base on a piece of cardboard, and figure out where you want it to go from the interior, making sure that the hatch won't catch on anything (especially once opened) nor require cutting away electrical wires etc.
Drill a hole through the center of the cardboard cutout straight up through the "ceiling" to the deck so you can then find the location of the hatch from the outside, and then line up the same cardboard cutout over the same hole from the outside (since the hatch is a symmetrical square shape, you don't need to keep track of the "inside" and "outside" of the cardboard cutout) and trace the outline for cutting the hatch hole on top of the deck.
I used hole saws to make the corner cuts match the hatch, but if you're handy with a jig saw you can do the whole thing using only a jigsaw. Because of the camber on the deck, there's a 1/4" gap around the sides under the hatch frame which, weather permitting, will have to be filled-in with shims and fiberglass filler to create a flat surface.
Having cut open the deck I was happy to see that it was more than an inch thick, and balsa wood in the core of the deck was in perfect shape. I dug the wood core out from for about an inch all the way around the interior of the new hatch hole using a chisel and oscillating tool, and then smothered the gap with filler and sanded it down smooth and flat. This ensures that no water can sneak into the wooden deck core from around the hatch hole to cause rot.
Screwing down the hatch will require good weather but I don't think it will be necessary to through-bolt the hatch. Things can't simply be screwed into the deck as water will intrude into the wooden core from around the screw holes and cause rot. I bought 8x 1.25" number 10 Philips-head stainless screws to attach the hatch to the deck but first I will drill out the screw holes to a larger size, fill the holes with structural fiberglass filler and put the (wax-covered) screws into the screw holes along with the filler. The wax will prevent the screws from getting stuck permanently into the filler once it sets but the fiberglass will take the impression of the screw threads well. The idea is to let the filler set around the screws to give them a real solid "bite" to hold onto the deck top when I permanently install the hatch, and also keep out moisture.