Treehugger
Hm, this works surprisingly well even if I simply hold it horizontally. I wonder how well the paint is mixed this way. Presumably the bottle will spin faster than the agitator ball inside. The 18mm masking tape also keeps the bottle still in place, it didn't come loose or snap (so far).

One could ofc just open the caps and stick the blender into the bottle, but then you have to clean up the paint smeared onto the mixing tool. I've seen the paint level in a humbrol bottle rise when using this thing inside it, I wonder if a thin Vallejo bottle would overflow when starting this thing up, inside a bottle that is fairly full of paint. Hmm. Outside shaking like in the image seems safer.

Treehugger
I don't know, I think I would *cough* prefer stirred not shaken. 🙂

I also found this product, which looks like a super shaker, but also with a super high price as well. 😄
gamesup.nl/product/paint-shaker/