Have you ever built a selfbow or board bow before? If not learn proper tillering first then you can build whatever you want. Trying to start out building what would normally be a difficult length bow to build for a 28" draw is just asking for frustration. If you just have to build it I would suggest getting the best piece of ash lumber I could find, one with perfect grain both back and sides and make a bow that bends through the bow from tip to tip (aka a bendy handle bow). I think I'd back it with rawhide, silk, linen or even brown grocery bag paper for a bit of insurance.