To get the profile right, I take an old prop like the one I'm going to use, and cut the hub out of it so it fits inside the spinner cone. Then I trace the outline of the blade profile on the inside of the spinner and cut it out. Once I have one side done, I trace this opening on card stock and transfer it to the other side, using it as a guide to make the other cut out the exact same as the first one. Use care to align the cutouts so they are precisely opposite each other.