I added one more row, so that when I'm using the last one of a colour, I can add a new pack of 8 behind it, plus two extra columns for different sizes.

I also made it 3mm taller, so that the brushes don't quite touch the surface it's resting on, and made the circular holes very slightly bigger as I found the manufacturer's moulding of some of the brushes was a little off and they got stuck.