mike is right,, its very hard to find just the right pc of wood  for a 1pc core,,i gave up finding that pc of wood long ago,,but I have made a core with just three pcs,the riser carved in ,the siyahs added later,, 
using just one  can be done ,takes a lot of looking and culling of material...  
p.s.  I do have an acient  crab bow, that the limbs and siyah are 1 pc then splices at the riser,,,hence    proven methods =proven results