Apple iMAC G5 Case Cooling Mod
 

To fix the heat issue I tapped into the existing ducting just under the plastic and mounted a Papst 120mm fan with a flow rating of 50 cfm to help solve the problem. Since the fan is externally mounted I chose to power it by an external adapter. After marking the spot I want to install the fan I drill the plastic very carefully, like a bowling ball almost, but I use a hole-saw chucked in a drill press...

Link: http://www.legitreviews.com/article.php?aid=227