Another thing you can do to aid the task of producing common cutout shapes is to make yourself a custom filing guide. The idea is that you spend a lot of time making your desired cutout in a rather thick piece of steel (1/8" = 3mm is handy), then after you remove the bulk of the cutout material from your actual project with a step drill or punch, you clamp the filing guide to the project and file the project metal away in the exact shape of the guide. The guide makes it so that you don't have to be at all careful how you file, since the steel will prevent the file from going "beyond the line" of the desired cutout shape. You start making the guide by carefully laying out the desired shape on your steel blank. Carefully chain drill the outline just inside the line, then file up to the line. Extra points for using tool steel and hardening it once you get it to the right shape. You could also ask a friend with a milling machine to make you one. It is a fair amount of work to do by hand, but you only need to do it once and it will last for many years, particularly if you harden it. For an IEC cutout, include the two screw holes so that you can use them to attach the guide to the project during filing.
Filing guides are also good way to make straight edges on sheetmetal. After making a rough cut with a hacksaw or cutoff disk, clamp a length of thick cold-rolled steel bar to your workpiece, aligned with your scribed line. Clamp the whole thing to your bench and go at it with a file. The guide will stop the file from going past the line, so you can be absolutely careless with the file while getting a perfectly straight edge. Once you have filed up to the line, draw file the edge to produce a smooth, burr free edge. I use a large piece of angle iron for this that I can clamp in a bench vise. I've added a clamping bar that uses bolts and tapped holes to secure the sheetmetal workpiece. You could also screw the angle iron into the edge of your bench to make custom sheetmetal vise.
While we're at it, one other trick for filing rectangular cutouts is to make yourself a "safe edge" file. Take a standard flat file and grind away the teeth along the narrow edges. This prevents the file from cutting sideways, and lets you easily file perfect square corners without the risk of the file damaging the side edge.
