I decided I wanted a vortex generator to mix my paints, but also didn't want to spend a lot of money. Fortunately this was already on Thingiverse: https://www.thingiverse.com/thing:3140367
The cone topper works great for mixing dropper bottles (cap-side down). But I have craft paints, Tamiya paints, washes and other larger bottles that also need quick mixing. I used calipers to take the dimension for the stem and built this extra wide mixer top.
I recommend test fitting this to the larger_bigger_coupler in the original files then use jb plastic to epoxy this topper to the coupler. That will help with issues of the stem breaking.
I had not used a vortex generator before this, so a couple observations: only press down as hard as you need to to keep the bottle in contact with the spinning parts. This is usually light pressure. Too much will create a lot of friction and can melt the dropper caps. Related, only squeeze the bottle as much as is needed to prevent it from spinning. Squeezing too much tends to also create a lot of downward pressure.
Print as oriented unless you want to spend time sanding away the connections for the support material. This thing spins fast and the plastic is already mildly abrasive.
I used 5 outer shells to ensure sufficient strength.