I recently picked up a used vortex mixer from ebay to mix my paints. The included adapter was great for mixing my 1/2 oz dropper bottle paints from Reaper, Army Painter and Vallejo, but didn't work for my extensive craft paint collection in the 2 oz bottles. No matter how hard I tried, I couldn't get the vortex to form in the larger craft bottles by holding it on the too small holder.
So I whipped up this design in Fusion 360.
This is in two parts - an adapter base and the actual craft paint sized cup. Print each flat, and glue together. There are three keyholes intended to use short pieces of filament to align the pieces and help hold the pieces together.
you can also print just the adapter base and glue a piece of mousepad to it to make a flat plate for other sized bottles. I may add further sizes if I find myself needing them.