I remember seeing a sourwood bow on the old PA board, seems like the guy who made it was happy with it. I don't see why it would need backing, sourwood is pretty strong stuff. I would make a standard design flatbow for medium-weight hardwoods. If you have a 28" draw, I would probably go 66" ntn, 1 3/4" -2" wide fades to midlimb tapering to 1/2" nocks. You could also try a 60"-62" bend-through-the handle Eastern Woodlands-style flatbow about 1 1/4" wide. It should work with either design. I have a sourwood stave kicking around somewhere that Pat B gave me a couple years ago, but haven't tried making anything out of it yet.