Take 2 flat mirrors, set them edge to edge at an angle of 90°, mirror side facing into the angle. Position the mirrors in front of you with the edge horizontal

Take 2 flat mirrors, set them edge to edge at an angle of 90°, mirror side facing into the angle. Position the mirrors in front of you with the edge horizontal and both mirrors angled at 45° towards you, i.e. look into the inner corner made by the 2 mirrors. The image you see in that corner will be flipped vertically. The same trick can be used to create a horizontal-flipping mirror, simply by positioning the mirrors so that the edge is oriented vertically. You can make one of these by sticking two pieces of mirror in an inside corner in a room somewhere.