My wife asked me for an adapter for our sodastream bottles for the car. Although easy enough to design and make, each of our cars has different diameter cup holders. I could not just download any model since they would not necessarily fit.  Thus, I would need to design and print a different model for each. Even then, there would be a bit of a wiggle unless the tolerances were perfect (as well as the measurements). Thus, the idea for this model was born. 

The truly unique part of this model is the base. It is designed to be printed in a flexible filament (in my case I printed it in TPU). This allows the holder to fit in multiple sizes of cup holders (i.e. you can reuse it between cars instead of having to print a new one)

The TPU base has a larger diameter for the cup holder on the top compared to the bottom. This is so that the two will fit together without glue or an adhesive. The small slits are present so that you can peel the inner ring back so that you can wrap it around the cup holder base. I used my finger, but you can also use a butter knife or similar long object to peel the inner ring around.