Very simple design I came up with to drain the bottom of the shampoo bottle into a new bottle. There is a small ledge on the inside for the bottom bottle. This will keep the shampoo from leaking out all over the bottle. This was originally designed for Suave Men 3in1 bottles, but I'm including the FreeCad file so it can be made to fit any bottle.

Prints well at .2mm layer height, with the inside ledge pointing upwards. Supports are needed on the inside, I also recommend a brim.