or you could just get a jigsaw and get a metal cutting blade (there only lke 5$ for the blade) and measure the fan and then viola a hole. (if your good the hole is almost perfect.
(also ive never cut a computer case so im not sure if it will be "not chewed")
if it's a 120mm fan, use a 120mm holesaw....
then you just need to make the holes for the screws, which you can do with a steel drill bit
or if the case is steel, maybe use a tungsten one
in my experienxce holesaws don't work very well on cases. they skip too much and take forever to cut. the cutting disc on a dremel works well but takes a long time. you cut a polygon out that is roughly the shape of a circle and then sand the edges until it is round
i have a punch set, i just dent the case where i want to, put the pilot bit in, and it doesn't wiggle all over the place. I've seen the work that dremels do, not really what i want to be doing with my time.
well its really your preference what you want to use, some prefer jizsaws, some like the dremel (myself i prefer the dremel) and some like using hole saws, if u want to use a hole saw use a 12cm one and be really carfull not to be on an angle or anything and make sure your using a metal cutting hole saw iv seen people flipping out tearing tehre case apart with wood cutting bits and not know why.
Hole saws work perfect for me and a dremel you have to be good and the stay on the line. Just lay down some masking tape on and around the part where youll be drilling to help prevent chipping of paint.Try looking for a 10cm hole drill and the hardware store with a ruler.The 120mm is the whole fan including mounts so you dont want to drill so big that you dont have mounting holes.