Osage, ipe, yew, and hickory are some of the better common belly woods, but everyone has their own preferences sometimes. It depends on what you're backing it with-matching the backing to the belly wood so that the back doesn't overpower the belly is probably more important than what woods you use. That 2" of walnut sapwood should make a good selfbow by itself. I have seen several good walnut bows made from sapwood, heartwood, or a combination of the two. Walnut heartwood in board form is often backed with hickory, ash, or something similar.
