I've made a number of very nice shooting self bows from walnut, but its a bit brittle, so leave it on the long side. The lighter, sapwood is best in tension, and the heartwood in compression, so the very best is a combination of sapwood for the bows back and heartwood for the belly, I split log is ideal, but you can do it with a fairly straight board. But I would recommend 68" tip to tip for a 28" draw, 66" minimum, and a pyramid design, ala TBB.