A sharp step bit does a great job on sheet metal if you keep it from walking around. I drill a 1/4" pilot hole with a regular bit, then clamp the work piece to a chunk of 2x4. Then I drill down through the metal and into the wood underneath. It helps keep the bit from wallowing around or snagging. A little de-burring, if needed, with sandpaper and I get a neat hole for tube sockets or whatever. I have been meaning to buy a set of Greenlee punches, but they are expensive...well so is a step bit, but I already have that.
Here are some final pictures:
