I set out this evening to make a cryptex, it was fairly ambitious given that I also had to pack for New York. Instead all I managed to achieve was a single ring prototype but I figure that is most of the hard work done. This cryptex features a ratchet to make the ring rotate in a single direction. 27 digits, I view the blank character to be equally as important as the other 26, and as this cryptex will be infinitely extendable you could have multiple words in the code. The central section is square which means it doesn't have to be made from 100s of stacked layers.
Most significantly there is an outer ring containing all the letters. The letters are glued into place using a jig to ensure they are all square on the wheel but the wheel can be rotated in relation to the keyed slots, this means the code word is actually going to be changeable once the cryptex is assembled.