ahh... in metal, i wouldnt use a hole saw.... how thick is the metal??

a jigsaw with a steel blade would do the trick, mark ur circle. drill four holes like north, south, east and west then run the saw between the holes. if steel is thick the blades wont last long but should manage the job. in the uk the blades would maybe cost £5 or £6 for a pack of four.

Draw a circle where you want the hole to be.

Drill in the dead center to create a tiny hole to work from.

From the center of the circle cut North, and then again from the center cut east, south and west. you should have 4 sharpe pices of steel still attachted to the rest of your box.

Now you can make your perfect circle a little easier...just gotta get rid of those steel flaps.

Its easier to create a perfect circle this way, instead of trying to cut the outside all in one shot...