I buy whole poles of bamboo and cut out 2 inch wide lengths, then run them through my drum sander to flatten the belly side. I normally get 4 backings out of one pole. Once the belly side it flattened, I trace my bows pattern on it and cut that out almost to the line on the bandsaw, then use a hand sander to get right to the line. Then I taper the backing from 1/8th thick at the riser to 1/16th thick at the tips.