Spy on those spying on you.
We had a piece of mirror that we wanted to use for a project. We decided to build an optical device that would let you see backwards. You say a mirror does that already? Well, yes. But it gets a whole lot cooler when you put it in a box.
Unfortunately, we don't have the device any longer. But the plans are below. We built the box and mirror supports out of cardboard. We attached the mirrors with double sided tape. Put your right eye to the left hole. It will see backwards while your left eye will still see forwards!
We used a tiny bit of math. If the box's depth is d then make the mirror and its support sqrt ( d * d / 2 ) wide and attach their ends d / 2 from the corner.