I didn't like the standard dopper bottle cap too much and wanted something I can stick a finger through, hook a carabiner onto, or add a cord to it. So I modeled a cap with a loop and a hole using the thread from the sodastream-dopper adapter from Bas (https://www.printables.com/model/108323-sodastream-dopper-adapter).

It includes an o-ring to be printed from flexible material. However, even without it the seal of the cap is pretty watertight, so that makes the o-ring optional. The o-rind has a notch, which should be on the direction inside the cap.

I printed it in PETG or ABS plus the o-ring in TPE (83A).

My first version of the model model was thinner and I broke it by screwing it too tight. This one is stronger, but best print with at least 4 wall loops and at least 4 top shell layers (printing with the top bit down).