I don't think it would really work that well. The stuff doesn't much like direct impacts, namely from your backside bouncing up and down on it in the waves, and it breaks down very rapidly in direct sunlight. Glass coating it might work though. As far as forming it, the stuff plays weird, it's pretty much either set, or not. So sitting on it in a box will most likely result in you sitting on the very bottom of the box, with foam squirted out everywhere. I'm not sure how one could achieve a good buttocks impression with the stuff. Perhaps hanging from something and lowering onself in only part way.

I don't know if it would really work anyhow. An exact buttocks impression would severely limit motion and result in stiffness and chaifing. You want some room for moving around a bit to stay comfortable. That's why a tractor type saddle works so well. It allows wiggle and squirm room.

> Using expanding foam for a seat sounds like a good idea. I would
> build a small box outside the boat, put the foam in, wait for some
> expansion, lay down plastic, sit on it (old clothes just in case),
> etc.