Flip the bed on its side and trace the outline of the side profile with the bump for the bolster. Add a half inch seam allowance on the top and sides and about 3 inches on the bottom. Cut 2 of these, and make sure that you make them mirror images. Measure the width of the bed and cut a section of fabric that wide plus 1" for seam allowance. use a fabric tape measure to get the length you need, add 6". Sew the sides to the top using a 1/2" seam allowance, then make a casing on the bottom, adding a draw string or really strong elastic.