Use this threaded ring to join two bottles together to transfer the remaining shampoo etc. from a used-up bottle to a new bottle. The threads match most bathroom use bottles. There is a collar inside to seal the bottles. I used it before I added the collar and didn't have any leaks but with the collar it should be better.
I always print a brim with 3 outlines 2 layers to make sure the head is running smooth. I make it 2 layers so it is easier to remove from the bed. I print on clean PEI with ABS bed 110 head 230.
I designed it in OnShape. I measured the thread pitch and size with a micrometer and ran a couple of prototypes to get the diameter right to screw it on the bottle. I added the collar on the inside with a bevel to seal against the top of the bottle. I don't know if the collar is really needed. To get the knurled surface I created a helix of 1/4 turn and created a new surface along the helix. Then I did circular copy to get the right number of elements around the cylinder. Then I did a mirror copy of all of those elements to get the knurled pattern then I did a Boolean subtract.