My dad give me one of these for christmas last year.
http://www.techist.com/showthread.php?s=&threadid=36843&highlight=DeWalt
Here's the part numbers for the Hole Saw.
#D180048 3-1/8 (76mm) Bi-Metal Hole Saw for 80mm Fan
#D180072 4-1/2 (114mm) Bi-Metal Hole Saw for 120mm Fans.
The whole saw is AU$40 ea
However you need to spend another AU$40 on a mandrel.
After that, you need a good quality power drill.
After cutting the hole, you would want to use a Power Filer to dull the sharp edge that is around the hole that has been cut.
If you do not have a AU$80 for the DeWalt Gear, then you can get a cheaper concrete hole saw. However I have been warned that since they are designed for Concrete, they might shatter if you use them on metal.
And even if they don't shatter, the blade will dull very quickly.
Otherwise, there is using a jig saw. Mark down your circle. Then drill a few holes with a power drill. Then cut the rest of the circle out with a jig saw.
Make sure you are using blades that are make for metal
Also where protective eye gear. Last thing you want is piece of metal getting stuck in your eye.
