Coosa is a fairly good material--it is PU foam with fiberglass fibers. The other materials I would consider is plywood and Nida-Core. The Coosa is in the middle between plywood and Nida-core by weight; about half the weight of plywood. A 3/8" 4' x 8' panel of Coosa varies between 15 and 24 lbs, depending on the type of Coosa (there are 5 different types, and thicknesses from 1/4" to 2" thick). Nadi-core for a 3/8" 4 x 7 panel, is 5#, so considerably lighter. But it is harder to work, and you have to seal the edges (even though since it is a honey comb material it should not adsorb water). You may have to use a little more glass on the Nida-Core. I redid the cockpit deck on my first (2003) C Dory25, with Nida-Core to keep it as light as possible.
Even though you can use screws on the Coosa and they will hold, I would thru bolt the hinges for the door.
What laminate schedule are you using for the Coosa on each side? What thickness Coosa?
As to making templets we used large sheets of cardboard (4 x 8 ) and a
Tick Board when building our 38 foot sailboat bulkheads. You can use 1/8" door skins also as templets and tick board. We would use the tick board method to lay out the bulkhead (which were 3/4" Marine plywood--they had substantial loads such as the chain plates of a mast over 50' high).
The bulkhead should be held slightly off the hull to avoid hard spots. We used trapezoid spacers made of a high density foam to stand the bulkhead off from the hull--only 1/4" is plenty. We then used thickened resin (micro balloons) for form a fillet on each side--used a tongue blade as the radius, you can use a slightly smaller radius if you want. The fillet gives a smooth transition from the hull to the bulkhead. Be sure and grind off the surface of the inner hull where you are tabbing in the the bulkhead--lightly with 24 or 36 grit with a grinder. You want a rough surface. The best bond would be with epoxy, next best is vinyl Ester and third is polyester. Most likely the polyester will be fine for the Coosa board. If using plywood, I would only use epoxy resin.
You then put the tabbing on. You want a wide layer first, then a slightly narrower layer. For example 4" first (about 2" overlap on the hull and bulkhead) of Mat, and then 3" wide cloth tape, (with about 1 1/2" overlap on the hull and bulkhead). You can put the narrower on first also, it is done both ways. I believe for the first time person the way I described is easiest. You can do this in short sections (about 18" to 24" where there is a marked curve.
Definitely want the door out of one piece! There are several suppliers of windows--the C Dory has use Diamond Sea Glaze--and those would match. There are others in Washington which are cheaper--I don't have the name of supplier off hand--someone else probably does.
You want to seal the edges of the Coosa around the cutout of the door and the edges of the door.
There are a number of different brands of door hardware. I have used standard Perko on many of my projects. They have a good selection of hinges and lock sets.
As for "sealing" the door. That is a bit more difficult. C Dory tried plywood covered (Decraboard), then Starboard, and settled on the Diamond Sea Glaze (or equivalent) glass doors). I would make the door out of the Coosa. I would then make a flange on the inside of 3 x 1/2 Coosa (or whatever thickness you are using for the bulkhead. 1 1/2" is adhered to the bulkhead all of the way around. The other 1 1/2" is available for the seal. (You might get away with 2" and 1"). Then a gasket material all around on the flange.
Finishing: I would use finish resin for the last coat (has wax in it) to make the surface hard. Let it dry, and then use boards or a long sander to fair the surface so there are no defects. You can make up, or use a marine polyester light filler for any surface defects. After fair, then roll on a coat of primer (sand lightly with blocks) and paint with Hatteras White (if a little dark/brown after drying, you may want to mix with a small amount of white) of Brightside Interlux marine paint. In the long run this is cheaper and should be very satisfactory, if you use a good bush and tip and roll technique.
