There would be nothing wrong with making a flat spot, under the skin, to potentially mount a rudder on. If you're building an untested design, it certainly couldn't hurt. If you're building a Yost boat, it's unlikely to need one, at least with the ones I've built, and by all accounts.

As for foot braces, I often use the frame by my feet for a foot brace - padding it with minicell foam first. I've also bolted on home made plywood footbraces on a few boats - they need not be complex if you don't mind not being able to adjust them. I attached them to the gunwales.

Another consideration is that you might not need foot braces at all. If I'm whitewater playboating, that's a different story, but I hardly bother with them anymore, and no longer feel a need for them. And yes, I do plenty of rolling.

A heel rest is a nice addition, to any Yost style SOF - I came up with this very simple one, and it is as comfy as a hammock - and it keeps my heels from contacting the skin (which, in cold water, can really chill your feet fast).

Good luck!

Dave Gentry

Heel rest: