Full instructable detailing assembly with an in depth guide on different bookbinding techniques:

https://www.instructables.com/id/Cryptex-Lock-Notebook/

To collect another nerdy hobby, I recently started doing some bookbinding. After tossing together a couple stab bound sketchbooks, I sought something a little more user-friendly (ie. a binding style that would open up laying flat). Fortunately, a few folks had already posted instructables on Coptic stitch bookbinding which seemed to meet my requirements:

https://www.instructables.com/id/DIY-Coptic-Stitch-Bound-Notebook/

https://www.instructables.com/id/2-LEGO-Journals/

Of course, being lazy, I wanted to improve matters find a way to create one cover that I could constantly reload with paper to avoid spending time and material on new covers every time I ran out of pages. Additionally, since my scribblings tend to be a bit subversive and my doodlings occasionally NSFW, I wanted a way to secure my notebook. Here is the result.

The 3D printed hard cover has holes in the spine for a separate thread that secures the stitches of the text block. On the front is a circular pattern cryptex with 4^5 possible combinations. When the correct one has been set, the locking pin may be slid down to release the latch.

Credit where it's due, I adapted a puzzle design I saw from Kagen Sound. He does beautiful work, and I highly recommend checking out his website.

https://kagensound.com/page2.html

Also, many thanks to Elizabeth Grab (www.elizabethgrab.com) for sharing her time and knowledge without which this project would not be possible.

Those that just want a cover without all of the cryptex tomfoolery can head over to my earlier, non-lockable design :

https://www.thingiverse.com/thing:3399102

On a side note, if anyone feels so inclined to tip me for my design work, please instead donate to the ACLU, Habitat for Humanity, Planned Parenthood, or any charity endorsed by GiveWell. Thanks in advance.