These let me stack paints in my travel box and not have to worry about them getting out of order or tipping too much.

The Citadel holders are remixed from SavageRodent's Customizable Rack. I made them hold six because that's the length of my box, but also split it into singles so that I can lazily c/p it in TinkerCAD for other boxes. Heck, you could just print a ton and plastic cement them in a row of as many as you like.

Vallejo holders are basically cylinders minus the helixes from AdditiveTech's Helical Gear Cups. They come in threes as that allows maximum configurability in my box, or single pieces available for your joining pleasure. These aren't plastic cementable as there are no good flat surfaces.