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
M3x8 bolts, socket head - 6pcs
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.

https://cad.onshape.com/documents/65a6b9e90c0ada0d5d47245c/w/21278f9fcc3d1733aa7a80cd/e/b7e7e917d359a9b0faf7745c?renderMode=0&uiState=6903cd9d2b3475b0aae1a7f7