The cylindrical lens inverts the image of object behind it. If you look at text through it with the cylinder horizontal, the letters and numbers are flipped upside down. If the cylinder is vertical, the text is flipped left to right. Some of the letters and numbers look the same when flipped, because they have reflection symmetry.

• cylindrical lens
• a paper with text printed on it

When making the paper with text on it, flip some of the text vertically or horizontally, so the lens flips it back the right way and makes it easy to read. You can hide a code inside some text, if you are clever. Laminate the paper if you wish.