Rob, Those monsters are called "Fly cutters" I think and they are very dangerous as the cutter is on the end of that arm and flying around in open air near your little blind body parts. Depending on the thickness of the sheet, you could drill a series of small holes around the perimeter of the needed hole and connect them with a hand held key hole saw.
Also depends on how clean a job you must have, could mean lots of filing There are circle cutting guides for hand held routers also and that might be a good way if you have the router and have access to the work area. Tom
