As a child I made several crypto disks out of cardboard, draw a 3 inch circle, give it enough segments to fit your alphabet, 32 is easy and gives you room for the capitals and some punctuation, draw the letters twice on each segment inside and outside of the circle, then cut out the circle, glue the outer part to a backing sheet and glue a tab/handle to the inner part to make it easy to spin.

Used statically it works as a simple ceaser cipher, but the real advanced ciphers became available if you rotated it by a specific amount after each letter.

Many plans and conspiracies about what we were going to do with our new "unbreakable" encryption system, but in retrospect, never used and more fun to build and think about than operate.