Nice project. I am going to undertake adding a cabin to a 19' Starcraft Holiday this spring/summer. I want something I can shut the door and keep heat in during the "frostbite" season here in Vermont, USA (Lake Champlain fishery has no closed season

and we fish year-round when snow or ice doesn't shut us down.) Plus, I want a dry place to unroll a sleeping bag and camping mattress on weekends in the summer.
19' is a little tight but the Holiday is wide enough to work out I think. As with yours, though, standing headroom will be tricky to come up with while making a priority of visual acceptability.
Anyway- wanted to 'bump' your thread as I have been reading your progress and hoped for more info from your last glassing question- and to read your questions are applicable to my project planning. Learned a lot so far! Looking forward to more pictures of your work.
My buildup will differ from yours in one major way as I am planning to mock up my drawings with cardboard at 100% right on the actual boat. Then I will rough out the cabin with 1/4" plywood over spruce 1x3 framing and 'glass itThen I will and fiberglass several layers outside and at least one inside right over that- window rough openings, roof, and all.