In my experience, those types of hole saws make a complete mess in sheet metal. Don't do it.

Really, just a set of (green or red) aviation snips is all you need. Useful for other stuff too, once you have them.

Eg. These: http://www.amazon.it/meccanici-meccanism...sr_1_13?s=tools

Just draw the circle on the panel with a marker or pencil, drill a 3/8" starter hole anywhere inside the circle, insert tip of aviation snips into hole, and start cutting.