To overcome the leak issue, I decided to use a replacement rubber gasket of hydro flask in my design. To test how much water tight my design was, I decided to start my designing a cover instead of going directly for the adapter.

hydro flask cover

Improve thread

I received a caliper as a gift that help me take more precise measurement of the hydro flask thread. This created a much more water tight interface.

Leak through the plastic

The first iteration I did had leak through the plastic itself. Here are some modification I brought to my design and printing process to get a water tight print

• Rounded edge inside the cover
• 3D print at higher temperature (210 C)
• Increase the number of layer on top, bottom and the wall.

Result

In the end, the final cover was tested with for 1h with the bottle tilted and couldn't find any leak of water.

I provided in the files section, the stl file of the hydro flask cover.