Tin snips, man. Just practice on paper or plastic first. Just like scissors, except they only go around a circle in one direction. The red one goes one way, the green cuts from the other direction. Don't touch the yellow ones for this job!

Practice is easy: just draw/trace a couple of concentric circles inside the real/final larger circle. Practice on the inner circles, then graduate to the outer real one. Has the benefit that larger circles are easier, too.

You mentioned earlier that you already plan to wrap the cut edges with something, so absolute perfection isn't needed. Black gaffer tape is an excellent edge wrap, if you don't want to bother with something more specialized.

Cheers