Ah Widget, the first C Dory I rode on!
As it happens, I am fresh from the experience of installation. I have a 2000 Day/night which I wanted to install. After putting it off for a week, I sat on the foredeck and thought long and hard about whether it would be the hatch or the deck behind the hatch. I called C-Dory to see if structure was an issue. They said no. A friend looked over my shoulder and urged the hatch, saying that if I made a mistake I could always replace that. I ignored his good advice because I didn't want to lose the light through the hatch. Also I figured if the boat was locked, someone could remove the vent or cover and reach through and unlock the hatch.
I measured and drilled a center hole, and scribed a circle. Then I went below and discovered to my horror that the interior was not shaped as I had remembered. The mating of the foredeck and the shelf over the V berth entrance is far forward of the windshield. Commited this far, I decided to move the vent slightly further forward, which would allow the 4.75 inch hole to end just in front of where the shelf starts. There appeared to be adequate room.
I took another deep gulp and started drilling and sawing. I had priced a hole saw (expensive) and bought a new keyhole saw (cheap). It took a lot of careful sawing. I managed to keep on the line (even if you strayed, the flange will be wide enough to cover problems). The hardest part was keeping the saw vertical, so the hole was same top and bottom. The hole came out fine, and the larger insert fit just right. To make the bottom insert fit, I had to cut the flange to fit - I cut it so there is a flat edge that matches the shelf edge. Then I gooped with 5200, and screwed it all in place. Came out perfect. The only bad part is that the 5200 might take a week to dry, which will keep me at home.
Conclusion: I am way glad I didn't put the vent in the hatch.
I'll show it to you when I am over your way, soon I hope.
Dave