If I were you, I'd make sure you're set on a certain speaker size, cut the correct sized hole, and to combat what Matt just mentioned (it's a very good point because even inaudible rattles cut down on volume and sound quality), make a ring out of 3/4" MDF. You'll mount that ring around the hole you made in the sheet metal, and then mount the speaker to that ring.
The idea is to isolate the speaker from the metal. It works even better with door speakers.