Pneumatic (air powered) cutters for light gauge metal. they are sometimes called air nibblers, put a hole in the metal big enough for the foot, and you can zip around the entire can in no time.
http://www.factoryauthorizedoutlet....y-duty-18-gauge-3-200-strokes-min-air-nibbler
http://www.factoryauthorizedoutlet....y-duty-18-gauge-3-200-strokes-min-air-nibbler