Paul, I like your idea of dovetails. Except for having to cut them. I have a old (1900 +/-)Wysong mortiser so cutting the slots is pretty simple. But I will keep that mind.

I lashed these in place and last night was thinking about it. I am going to cut out the lashings and glue them then relash them. If a lashing fails it would hard to go back and repair it. Since these are above the skin and not held in place by it I think it's a good idea to do belt and suspenders on these.

Yes these may contact the skin. Not sure that is a problem though? The skin touches the frame in many places but there isn't any movement. I don't see any movement or rubbing on these. All I see happening is the skin may be pushed against the slats but not rub. Of course I could be wrong but I am not seeing any abrasion there.

Scott, my slats are 3/4 x 3/4 (more or less). I didn't want much bending. Didn't want them deforming the bottom. As for the seat this is wider boat of my design, not one of Toms designs. I am going to try a happy bottom on top of this and see what happens. If it doesn't work out I will have to back and up and try something else.