I have wanted for a long time to put such a thing in an upper corner of my room, so that if you look at it from anywhere, you'll see your face upside down. 1-ft mirror tiles, available on the Web from various suppliers, are just the thing, but I worried about how to mount them. Then it occurred to me that they wouldn't have to be mounted on the ceiling; they could rest on one of my tall bookcases. So I ordered some, and put them together with duct tape, in such a way that the bottoms & tops of the vertical mirrors are flush. The top mirror, being unsupported at the outer corner, sags a little, so that it looks as if you had four eyes; but an inconspicuous strut, consisting of a 12-in. stretch cut from a coathanger, takes care of that.
The tiles come in boxes of six, so I have three left over. Anyone in the vicinity who wants to make a retroreflector is welcome to them.