To gain enough headroom, (I am 6'3") I have cut the corner of the cabin top at a angle. That means, the cabin top will be 8' wide, (again, little cutting to be made).
The cabin's beams, I think will have to be curved. A curved roof looks more boaty, is stronger, will permit to stand on, and use the cabin top. I think they will be laminated out of 1" thick planks. Having said that, it's a lot of work, and I'm trying to think of a way to keep them straight, but still look like the cabin roof is curved...like a "proper" boat! ..ummm. Keeping them straight could save me a lot of time and work.