Do you want to close your dopper bottle while using the original cap as a cup for drinking? Or do you want a smaller cap or need a replacement for the original?

It is challenging to make a 3d print water tight and foodsafe with regular fdm printers, but this seems to be working in both areas. Do not drink from it though, it ist just to keep the bottle closed.

1. Print the cap in itself with any hard plastic (I used PETG)
2. Print the seal in soft plastic (I used TPU 95A, but softer could work better)
3. Cut a circle the size of the seal from any reasonably thick plastic (I used the the bottom of a bucket of cherry tomatoes I scavenged around the house
4. Assemble  - put tpu seal in the cup followed by the cut plastic circle.
5. Put the assembled cap on bottle and tighten reasonably.

The tpu squish should provide water tightness while that added plastic ring should prevent the water to come into contact with printed parts. Can be detached and cleaned if needed.