A newer, fully redesigned version of this project is available:
Modular Magnet Organizer v2

Modular Magnet Organizer

A modular magnet organizer. So you can store your magnets without chaos and only have to print the ones you need!

Modules for magnet with size:

20mm, 18mm, 17mm, 16mm, 15mm, 14mm, 12mm, 10mm, 9mm, 8mm, 7mm, 6mm, 5mm, 4mm and 3mm

Read first

It is intended that the parts hold together firmly. This also applies to the magnets in the slots.
So nothing unintentionally fall apart or magnets fall out (regarding magnetic force).
 

Assembly

Carefully join the parts together with a small hammer or vice.

Notes:
Many of the models already available online, are made for magnets in sizes that you may not have, or you would like to add other sizes later.
So I created this modular model, with individual modules for different magnet sizes.
This way, you only have to print the ones you need. The modules can be joined together and the two ends provide a nice finish.

The first printing plate contains all the modules. Simply remove the modules you don't need, before printing.
The second plate is for the two end plates. This means you can easy print them in a different color or material (without waste).
Of course, there is also the option of printing the individual modules in different colors. STL files are also available for individual print.

For some sizes there are modules with single or two slots. Also a version of a module, combined for the 6 and 8mm magnets.

The modules are 50mm in height. If ouy need to store more or less amount of magnets, I think you should be able to scale in Z axis.

The modules fit tightly into each other. This ensures that everything holds together well.
I recommend using a small hammer for the assembly. Carefully tap the modules together with the hammer. The modules can also be separated again, using the same method. For example if you want to insert another size between them later.

I printed the modules in PETG and the ends with PLA. However, any other strong filament should be possible.
For a little more stability, I used 20% infill for the modules. Iron the top surface, for a beautiful finish.

The magnets fit relatively thight in the slots, which ensures that they don't fall out so easily.
With the labeling, there is also a little more space between the magnets, so the do not interfere each other so strong.

This is my first upload. Please let me know if you have inputs, problems or request for adjustments. I will gladly try to help.

Additional notes:
There are also STL files for a loose-fit version. (the male side of the connection is slightly smaller)
If you are not sure, whitch version gives the best result for you and your printer, probably try first and print one single piece from each version. So that you can test the joining of the parts.

The idea is to have a tight fit, so the parts cannot fall apart. The thight fit is by purpose and you maybe have to use a hammer to put them together.

If you would like to have more space for the magnets, you should be able to scale it up, so it fits better.

I have uploaded some experimental STL files. They are not testet! Use at your own risk!
Please no rating, if you have any problem with this files.

There is a "deeper" version for the 3mm module.
Because some people have mentioned that the magnets do not hold.
 

And 1mm, 2mm, 25mm and 30mm modules.
There were requests for these sizes. Depending on the tolerance, it could be difficult for the smaller sizes to work. The large ones stick out on both sides. If that doesn't bother you, you can use it. Maybe use more infill, for better stability.
 

UPDATE, 13/06/2024:

• Added STL files for a 16mm module (orignal and loose-fit)
• Added STL files for a 32mm "experimental" module (orignal and loose-fit)

UPDATE, 21/04/2024:

• Added STL files for a 9mm module (orignal and loose-fit)

UPDATE, 17/04/2024:

• Added STL files for a 14mm module (orignal and loose-fit)

UPDATE, 24/03/2024:

• Uploaded some experimental STL file. They are not tested! Use at your own risk!
  Contains: A "deeper" version of the 3mm module. And 1mm, 2mm, 25mm and 30mm modules (see notes below).
  Changed 1mm and 2mm a bit regarding some feedback (v.1.1), also added loose-fit version of them.

UPDATE, 17/03/2024:

• Added a 17mm module (orignal and loose-fit)
• Added brim for the two end pieces

UPDATE, 15/03/2024:

• Added a 7mm module
• Added the STL files and a separate plate for a loose-fit version (see notes below)

UPDATE, 11/03/2024:

• Added a 5mm module