Either I have been very lucky or the above posters have made this task far more complicated than I needed. If they already have the tools, that is good use of them. I have had to do it this way because I don’t own some of the sophisticated cutting tools mentioned (lathe, Bridgeport). My only experience is with Sinar aluminum 5 ½” or 14cm boards. I have drawn the size hole needed on the lens board, clamped it in a vise using wood on either side for protection, drilled multiple smaller holes well within the drawn circle, used a saber saw to rough cut a circle smaller than the one needed, and then an appropriately sized sanding drum on a shaft in the drill to grind out to the size needed. I have checked often to be sure the circle I am cutting is round and within the size needed. As with hair cuts, one can always cut more, but never put back.
When I first needed a hole cut I checked a local machine shop and found a basic set up fee of $75. Some of the specialized photography machinists were more expensive. I bought a set of sander drums for about $10.00.