3D Grill for Secret Writing
The grill consists of a square of paper in which holes have been cut in a certain way. Suppose that the paper consists of a square with four smaller squares cut out. Place the square on another piece of paper and write the first four letters of some text into the holes. Then, rotate the grill a quarter turn and write the next four letters. Repeat two more times. This Demonstration shows such a grill, which can be 3D printed.
 H. Lysing (J. L. Nanovic), Secret Writing: An Introduction to Cryptograms, Ciphers, and Codes, New York: Dover Publications, 1974 pp. 36–37.