I just drilled through the top of an upright freezer, and went real slow to make sure I wasn't going to hit anything.
I started by using a dremel with a guard (I think it either came with the kit or was part of a grout clearing set). I used a small drill bit and popped a tiny hole just far enough to get through the metal. I then used the bit to keep popping little holes around it to make it large enough to poke around in the foam. After clearing enough foam from this 1/2" hole I then used the same guard and the general cutting bit (book says it's good for a few things including aluminum siding) to make the hole about 1".
At that point I continued to clear out foam with a sharpened chopstick and shop vac'd the loose stuff out until I was sure that there was nothing in the way. I measured the depth of the hole and determined that I was at the inside wall. Using a hole saw I cut through the last little bit and I was in. I then used the grinding stone to clean up the edges so they weren't sharp.
It all might have been a little overkill, but I'd rather spend an extra 20-30 min and not have a very heavy piece of trash to get rid of, and have to start over on a new freezer. I can post the part #'s for the dremel pieces if it would help.