Just curious about something, since there are a few of you posting about cutting openings in cases.
Have any of you thought about just calling any local heat and air shop that does sheet metal work?
A fan hole should take an experienced sheet metal guy about 5-10 mins tops, including leaving an acceptable smooth edge.
I have a farly extensive background in HVAC (including sheet metal fabrication). To me it just looks like another hole in metal. Just mark where you want it, grab the snips, cut the waste out (slightly undersized), grab the die grinder (or dremel) and smooth the opening edges. Done! An experienced tinner should easily be able to cut a 2' hole with his snips and leave almost no distortion, just a few burrs.
BTW That is one of the reasons I recommend a nibbler They will easily cut through metal 50% thicker than most cases and leave an edge that needs almost no clean up. Plus they don't require any experience to get good results, and there isn't any need to be careful that your workpiece doesn't slip /spin.