All sound techniques offered by the first posts! Main thing is to be patient and don't be aggressive with the rotary tool. I bet I trial fit my cowl 10 times as I get the main cylinder hole cut to final size. I use the metal bit that is shaped like a cone to rough cut the holes, and then the sandpaper drum bit to finalize the large holes. I normally turn on my central vac and have the pointy nozzle a few inches from the rotary tool business end. I wear the ear muff hearing protectors to save my hearing from the vac and rotary tool noise. It's actually a lot of fun when it turns out right! Jon