I followed Steve83's routine for installing an access plate. These pics are of the helps I used after reading his warnings on cutting too deep. For the whole routine check out his info here: http://www.supermotors.net/registry/2742/33276-2

First I stacked nuts on the drill bit to make a cheap stop.

Then tape to hold them in place to drill out the pilot hole locations.

A hunk of PVC cut to length for a cheap stop for the hole saw to drill out the corners.

PVC stop taped in place.

I used a cordless center cutting shear to keep from cutting too deep. It worked easily on the flats, but took a little rocking to get up the ridges. I would think a manual version might work a little better, since their jaws are usually a little deeper.

The center cutting shear leaves a narrow strip that curls up as you cut. It allows you to cut into the middle of sheet metal much easier than using traditional snips.