Might be a bit heath-robinson but I'd just drill a hole big enough to get a jigsaw blade in, use a jigsaw to cut the bulk of it, then a round file to get the hole just right. I've never thought it worth buying a hole cutter for a one-off job like this.

Ages old now but for anyone else looking, i'd reccomend a tool called a tank cutter.... http://www.plumbworld.co.uk/hexagon-shank-adjustable-1822-1537 really handy thing to have as they can make any size hole. When working metal go very very slowly, spray oil on constantly and keep checking how hot things are getting. Work the hole from both sides (or at least score from one side before cutting all the way through from the other) as this prevents any breakout/excessive burring or indeed taking off flakey paint. You should literally be looking to spend about 30mins making a hole through sheet steel but it will be spot on :)

 you could use some wood screwed through the potential hole to brace the area and stop any flex. i used an angle-grinder with cutting disk to do the hole, then a flapper disk to bring it down and eventually a round file and sandpaper.

good luck, its scary cutting holes in a new roof.

matt

The Enox range of hole cutters are ideal for the purpose, pick one with fine pitched teeth and just take it steady. Drill a pilot hole in the centre of where you want the hole & ease the spinning cutter down onto the metal. A little oil smeared on the surface should prevent the cutter grabbing. Finally, ensure there is nothing structural or wiring below where you want the hole.

Stearman65.