Well, ultimately I did what a real man should, that is, ignore everyone's advice and just go with my original plan.

Nah, there are pros and cons in all of these options. Nevertheless, this time I still wanted something matching the list given in the first post. It turned out that the solution was indeed something roughly jewellery-related. One can get started with keywords "bead container", which yields some multi-slot organisers but also single boxes. I bought two different deals of those, one with square shapes and another with non-square rectangles. They look like this. Assorted junk included for scale.

I find those comparison items more informative than pure dimensions. Having only the latter in product descriptions was one of the reasons to get several sizes for testing. Anyway, the sizes are (in mm):
Square
- 53 x 53 x 20
- 63 x 63 x 20
- 73 x 73 x 25
- 83 x 83 x 28
Non-square
- 55 x 40 x 20
- 85 x 55 x 25
- 115 x 85 x 28
(According to the seller. The exact dimensions may differ slightly but those sound close enough.)
Other sizes probably exist too. For example, you can also get 180 x 110 x 30 from the same seller but that felt quite large already for my purposes. These should be "desk sized", so that I can make modest amounts of custom mixes, keep them on my painting desk, and ideally do the sprinkling right over of the same container, still without wasting too much space. Of course, using a larger one like a takeaway tub as a "work unit" is totally possible too. Meanwhile, I also got a bunch of sturdy ziplocks for storing the excess from large bags more safely.
I'll start experimenting whenever I have free time. Let's see which sizes really work, how well they stay closed and so on. I may keep reporting if you're interested.
