Torjusg is quite right, seats aren't something native American used. I have tried to do without them on a couple of canoes. The problem I had is that I wasn't born in that culture and my knees were really not used to kneeling for long periods I suppose

and it was getting quite painful! (and don't mention my age please!!
So if you still want to do without, good for you! The way I would do it is to fit the batten on the inside side of the ribs on both sides, cut the seat battens to fit on the inside of the ribs, drill a couple of holes in the battens and put pins that would lean again the inside of the battens, then to tie the seat quite tightly to the battens. does this make sense

If it does not, i'll do a drawing and post it in the next couple of days.
In fact, you could also put the pins through both the seats planks and side battens use some hardwood such as oak and they should not break

; if the battens are fixed flexibly to the ribs