I have used a rat-tail file to cut the propeller openings in an aluminum spinner. That is probably the best way. It’s slow going but it works. I have also used a dremel tool. The dremel is faster but you have to watch the speed and the tool (too fast, it burns, too slow it chatters). Use a larger diameter burr, and don’t turn it too fast. If you use the dremel, mount the spinner firmly and don’t hold it in your hand. For final fitting be sure that the spinner cutout doesn’t touch the propeller when it is mounted on the engine. It will notch the propeller blade where it touches and can cause the blade to separate with serious results. Take your time and work carefully. It’s not difficult, it just takes patience.