I'm un sure which hole you are talking about. The fan I made doesn't require any drilling of the case at all. It grabs the "ridges" on the front of the fat PS2 to attach it to the case. Although I did drill holes in the case underneath where the HDD sits, for better ventilation. See the pics posted in this thread if you are intrested.
http://www.ps2newz.net/forums/showthread.php?t=47706