If I have this right, you're trying to put a wooden (probably) frame around a mirror which has 2", 3" etc. probably bevelled mirrored strips around it? If so, and if the frame you're trying to put around it has a deep enough rabbet, just drop the mirrored article into the regular frame, back with either gatorboard or foamboard, secure with points or heavy-duty staples. I don't glue anything because inevitably someone wants it undone later. I've found no difficulty with the above method unless the thing is gigantic. I also cover the back with hardboard screwed to the larger frame: this reinforces the package tremendously. Don't forget to paint the inside lip of the overframe so it's compatible with the reflection of the mirror pushed up against it. Add three hole mirror straps on the vertical sides and that should do it.