Mine was loose also. It looks like there is thin plywood behind the mirror with the frame around all. I just drilled additional holes and installed very short stainless steel sheetmetal screws with the sharp ends ground down. I could feel them just touch the mirror when I screwed them in. I don't think too tight against the mirror is a good idea and especially not with a pointed end pressing against it. I drilled the holes then pressed the frame together and installed the screws. I used 2 screws on the vertical and 2 on the horizontal frames.