This includes parts of the frame that the panel may come into contact with when sliding off the case. We get asked this question almost daily. While it does come down to personal preference we simply prefer the jigsaw for cutting window holes. A dremel or angle grinder is fine for cutting fan holes but, for us, a jigsaw does the job cleanly and easily in around 10 minutes. That being said, there are many people out there who swear by the dremel, and do a great job with it.
Some people even do a good job using a drill or holesaw.
We just prefer the jigsaw. If you don't own a jigsaw or can't get hold of one from a friend, try the local hardware stores as many hire them out. Arrange the molding around the hole, working the side of the case into the smaller 2mm gap. Make sure the ends of the molding are at the bottom of the cut. Once the molding is in place, take the window and work it into the rubber opening.