Need to install AC vent "balls"

[Tallguy]

I've seen different posts with members using various methods to layout holes to cut in dash for AC vent balls and was wondering what works best. I'm leaning towards using the actual factory vents that have been trimmed in back to lay flat on dash to draw an outline. The center vent has already been installed correctly, but square holes were cut in each side of my dash where someone put in vents from an 86 Buick or something. Any thoughts? Mike