This is a modification of the original Star Trek Adventures Tricorder Dice Holder made by Crossman:

https://www.thingiverse.com/thing:4923767

The original held two D20s and four D6s for use with the Star Trek Adventures roleplaying game by Modiphius. In Second Edition, they have removed the D6 Challenge Dice and so, Al Spader, one of the writers on the game suggested that a version might be made that held five D20s instead (the maximum rollable in the game). I rose to the challenge and this is the result, with the consent of the orginal creator, Crossman.

It's designed to hold five standard sized D20's which are usually around 20mm in diameter, measured face to face as opposed to point to point.

NOTE: Some of the official Modiphius dice such as the Klingon and Kirk Tunic dice are up to 22mm in diameter and so it's worth measuring the dice before you print.
A simple solution for 22mm dice is to print the Tricoder at 110% which will make the sockets large enough to accomodate 22mm dice.

The magnet sockets are designed to hold magnets that are 6mm in diameter and 3mm thick. These are pretty standard and easy to find, I think.

The whole thing was modified in Blender and the test print done on a Creality Ender 3 Pro and printed perfectly fine with no supports.