This cryptex tube should hold a set of seven D&D dice nicely. The inner tube is 28mm in diameter and has an overall inside length of 182mm. If your dice are a little bigger, you can always scale it up a little. As designed, it will just fit on a Prusa Mini (180x180x180mm).

The cryptex has seven dials with faces marked 1 through 8 (count the vertices). I've provided eight different dials, each with the unlocking slot aligned to a different face. Stack up the dials in any order to create your custom 7-digit combination.

Printing
Print in PLA with 0.2mm layers. Use 3 perimeters and 20% infill. None of the parts require additional supports, but note that the Outer piece has a built-in break-away support for the upper lip overhang. (My Prusa Mini printed the overhanging lip fine without the support, if you want to try eliminating it in your slicer.)

Print one Inner piece and one Outer piece. Before printing the dials, pick your 7 digit combination and then make sure you print one dial that corresponds to each digit. (The inner notch on the dial lines up with the associated digit.)

Assembly
The top end of the Outer piece flexes together enough for you to slide each ring onto the split tube. As shown in the last photo, the larger ID side of the dial should slide on first. Stack the dials up in the order of your combination. (Note that once you've inserted the Inner piece, the sides can no longer flex inward.)

Use
The “1” marker (the single dot) on the end of Outer piece is the indicator for where to line up your combination numbers. Dial in your combination and then insert the inner piece. You may need to wiggle each dial a little to get them perfectly aligned for the Inner piece to slide all the way in. Then spin each dial to securely lock the Cryptex shut.

To open, dial in your combination again and try pulling apart the two ends. Again you may need to wiggle the dials a little to get it to open. A lubrication on the notched rib of the Inner piece will help it operate much more smoothly.