Here's how I did it :
Get your fan grill and trace that outline with pencil onto your case. Then take tape pieces about 1/2" long and put them all around the circle (it will look like a ninja star with all the tape pieces on the edge of the circle, sweet) so you don't scratch your case when doing this.
(Remember to use goggles! Lots of metal will be flying).
Get yourself a Dremel or some other cutting tool and use some carbide blades to tear into that thing. Go slow so you don't tear the hell out of the bit.
Have fun!
-silver