Your best bet is fork over a few bucks and buy a hole saw for your drill if you don't already have one. Home Depot or other hardware stores caries ‘em - cheap ones too. First, cut a plywood insert that fit’s inside the pan. Reason: when you drill the holes, you will also drill into the wood which will keep the shape of the pan and prevent the metal from bending under pressure. It also acts as a “stop” for the cutter - when you see sawdust, you know you're through. As you drill, be sure to drill at a medium/slow speed while squirting a little gear oil on the cutter. This helps to cut down on friction and prevent the metal from tearing thus giving you a nice clean cut.
Trust me, you'll be happy with the results.