I used a Dremel to make the initial cuts and then a fine tooth metal blade on a jigsaw.
Could have done the whole thing with the Dremel but I found after a while being handheld at an angle I started to loose a bit of control, jigsaw was easier as obviously your working from above and the case is taking the weight off of your hands.
I do know a friend who cut the holes in each corner of the panel he was to cut out with a standard size 10 metal bit on a powerdrill, which is just big enough to get the jigsaw blade in to and started off.
Just take your time, let the tools do the work and dont force/push whatever your using (will make a hash of it and end up bending the metal around your cut) and spend a few minutes afterward with a file to remove any sharp edges