If you want to stay HYDRATED during parties or fesitvals where they take the cap off the waterbottles, this print offers a great solution! Fits on the Spa/Coke and Chaudfontaine bottles often served at European venues. Passed the test on watertightness!

• Shouldered internals for watertightness 
• Strong easy to print threads
• 4 different shapes (recommend the octagonal one)
• Optimizations for 3D printing

I tested it to make sure it doesn't leak when using PETG or ABS+ with a 0.2mm layer thickness, as long as you screw the cap on tightly. But be aware, PETG can be trickier because it tends to create strings and blobs. So, you should make sure to clean the threads and the inside part carefully. Also, PETG is a bit harder to screw on or unscrew because it's more "sticky" than ABS+ and thus has higher friction. 
Your milage may vary with other materials but the model should be more than sufficient for sloshing and holding it upside down with no to minimal drops.

• ABS or PETG
• 0.20 Layerheight
• 3+ Perimeters
• After printing check if threads and internals are clean

You can find a STEP file if you wish to change or personalize the design.