I could go that route but I'm probably too much of a perfectionist. My current plan involves having a cpu heatsink + fan combo that will only just fit the inside the height of the case (3" tall). Then I'll cut a whole in the case top immediately above that of the right size and add a minimal fan grille (just enough to mostly protect against chopped fingers as I don't want to limit the airflow). So I really want a neat hole that I can dremel a bit to remove sharp edges left after the hole is cut.
to save you money, how about just cutting a complete square/circular hole(forget neatness/precision, just making it a little smaller then ur intended size) then cover the hole with a plastic/metal fan grille (of larger dimension) from like(jameco.com) Hole cutters are expensive, for the little task you want to accomplish.