My GF needed a sturdy and stable paintbrush holder so I made her an over-engineered one that could probably survive a nuclear melt-down (it could not).

I also bought a set of heat-set inserts (see side pieces) and really wanted to try them out so there's 10 that you'd need.

Everything prints flat, absolutely no supports needed

For the hardware:

M3x8 bolts, button head- 28 pcs
M3 heat-set inserts - 10 pcs
 

Just screw everything together.

The tolerances on bolt recesses might be a bit tight, you might need to cut off the little pieces that lie on the corners of the top pieces.

The organizer houses 54 brushes, the openings are as such:
 

12x12mm - 16
10x10mm - 18
8x8mm - 20
 

There's a model on on-shape, feel free to edit it to your needs.