This is a ~7ml paint bottle that I designed for my mobile painting set up.
A standard bottle of Army Painter is 18ml, more than you need for painting generally, so I made these little half size bottles for ease of transport.
The use a standard soda bottle caps, which are called PCO 1881 by the ISBT (International Society of Beverage Technologies). These are the caps on pretty much every soda bottle of all sizes in the US.
They stack together tight enough that they can be picked up together but are very easy to separate.

I suggest using a support interface layer with these of about .6mm, it makes removing the supports very easy. I also used a support blocker on the threads since they don't need them. The reason the bottom is cut out is to maximize stackability while maintain a height that fits the box these are carried in. A single bottle is 32mm high but a stack is only 60mm high.