When I saw the cryptex Instructable (http://www.instructables.com/id/How-To-Make-a-Cryptex/ ) I thought: "I will make something like that with my grandson this summer to generate coded messages". He is smart with his hands and he will love building one. I also realized that if we were to play spies, I should have my own… I did not want to have one that he could compare with his, so I decided to make something quite different. Since I have some experience with sliding rules I decided that my machine would be flat.
Source: here