I take the spinner cone, base down on newspaper, and make a paper mache mold of the cone. Just strips of newspaper wetted with flour, water and little wood glue, about 3 layers thick. Let dry, trim, spray with PU to stiffen. Now with a mold of your spinner cone, you can mark, cut and experiment on prop cutouts. When you have it right, slide back over cone and mark your cutouts. Dremel tool with diamond cutters, sanding drums will finish the job. Make sure each cutout is the same for proper balance. This is the method I use, I am sure there other ways also.