Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:2058501.

My wife uses Lush 16.9 oz shampoo / conditioner bottles. When one is getting low I would transfer the contents from one bottle to another just by balancing them on top of each other.

Now I can just screw both bottles together and walk away while they combine together.

For anyone wanting to take the original file and remix it for use on bottles that have rounded threads (image included) make sure you add 1 mm extra to the diameter.