This is an adapter to hold one bottle over another to empty the last drops of 3D printing resin. I remixed the bottle coupler to better fit the Elegoo resin bottles I was using. I took measurements from a bottle using digital calipers. You can see my professional quality hand drawing I worked from. It is simply the dimensions of two bottle caps fit together.

Printed on an Elegoo Mars in Elegoo standard resin. It is tilted 45° and sliced with Chitubox 1.9.0. I put it on a raft on medium automatically generated supports with modified density and angle. The details are in the images but are not really critical; it is just a hole with threads.