I`d put the fan on the inside of the rear panel in the center and make a door for it with a magnetic latch to hold it open or closed. You will also want it to blow out of the panel as if you blow onto the mirror, you will end up with a cold center and a warm edge on your mirror as it cools. If you`re worried about the hinge getting dinged then you may want to recess the panel and the open area for it. Use a steel door and you just have to put two little magnets into the wood for the open and closed positions.