Disclaimer: This post contains an affiliate link to a product we bought, then promptly printed a version we liked more. But go buy one via the affiliate link anyway–it’s not that it didn’t work, it was just held too many brushes.
We needed additional brush holders that we mostly like this one we bought off Amazon:
It actually holds too many brushes for what we typically need, so we thought we’d spin out a customized one all quick-like, and use some 1/2″ ID copper tubing for supports. Since it wouldn’t be too wide, we figured a single, central support pillar would be sufficient (it is, but we also filled one with sand capped with silicon for extra heft just in case).
Here’s what we ended up with:
Front view, loaded 
Side View 
Front view
Design
We whipped up a screencast real quick that covers the bulk of the design process. IRL we parameterized everything so we could use other support material (like dowels or acrylic) and change the wall thickness (these are 1.5mm which is a little thin, but worked well on our MakerBot Method). We didn’t parameterize the number of slots; we’ve had mixed results doing that in Fusion 360.
TL;DR: Make a hole the size of your supports. Make the circles for each “row” of brushes you want. Draw a line from the center point to the edge of the outside circle. Offset those on each side by 1/2 the wall thickness you want. Make a circular pattern of those.
Then extrude up the height of the holder (we used 1cm). Duplicate all this for the bottom, and extrude around 2mm so the base is solid (keeps those drips off your table!) and you’re basically done. We filleted and chamfered everything as well.
Lessons Learned
This was about a four hour print on our MakerBot Method with the default settings. Faster than Amazon Prime delivery, and it suited our immediate needs better.
Overall we’re pretty pleased with how it turned out, although a little more slop for the support piece might have been beneficial–it was a tight fit, but the dimensional accuracy of the Method saved us from all sanding whatsoever, and the PLA it shipped with was plenty strong to stand up to a bit of pushing.