Probably the most foolproof method for a typical home user would be a bimetal holesaw. Guaranteed size, guaranteed roundness, guaranteed position. If you do this to a panel you've removed from the case, be sure to firmly clamp it down using wooden blocks and clamps, so it can't spin on you and injure you.
I've used fine-toothed sheet-metal blades in a small electric jigsaw. It's not too hard to cut a 120mm hole, but the tight radius of the 80mm holes made it pretty hard for me. Some hand-finishing with a hand file helped it look nice and round. You could accidentally cut too large of a hole this way, however, and then your face will look like this ---->