I used a 45 watt laser cutter, which I have access to at TechShop RDU, to cut parts for this design. I also used new paintbrushes because I don't paint very often and didn't have many old paintbrushes to make use of for this, however I think it would look good with some old paint covered brushes.
Things you'll need:
Paintbrushes - I used 17, any less and I think the cup would have ended up too small.
1/8" plywood - a six inch square piece should be plenty.
Calipers - for measuring the diameters of the paintbrushes.
Masking Tape (or painters tape) - for marking heights / identifying individual brushes.
Laser Cutter - available at your nearest TechShop.
Rubber Bands - Optional, I used them to hold some of the looser brushes in place in the final product.
To do this, I laid all of the paintbrushes out in a line and made sure the ends were in a straight line, then I placed a long piece of tape across all of the brushes and cut it between each one.
This also marks the specific height on the brushes where you want the top brace disk of laser-cut plywood to end up. I picked a spot on the brushes that was just below the fattest part on the shortest brush, so that I could easily slide all of the paintbrushes into their respective holes.
Once you have cut the tape for each brush, label them with numbers or letters.
Now you will measure the diameter of each brush with your calipers.
Each brush will have two measurements: one at the top of the brush (where you marked with tape) and one at the bottom (I took mine at about half an inch from the bottom tip).
Make sure you record the measurements with respect to the number on the tape.
(note that if you have rectangular brushes like the one I used you are measuring its width and thickness, not diameter, so it will have two dimensions)
Do not remove the labels from the brushes, as they will be important when you are putting the cup together in the final step.