It wouldn't be that hard to make another template from the mirror that is 2" larger. Just use a combination square or a pointed dowel against the mirror at a 90* angle and follow around the mirror at 2" farther out. You would have to noticeably deviate from 90* for it to be noticeable on the frame, something on the order of 10*.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.