This prints really easily in PETG or PLA. I have confirmed that it work son 2L Pepsi and Coke bottles (USA). I have not tested other brands.  

print solid. 3-4 outside walls

Directions

Fill one 2L bottle ¾ of the way with water. Add food coloring if desired.  Twist on Tornado connector, and then add an empty bottle to the other end.  Flip the bottles so the water is on top and give it a stirring/twisting motion.  A Tornado/vortex will appear as the water empties into the bottom bottle.