It turns out the caps of Aquaflask bottles have a weak point such that the inner stub of the cap holding the thread can break off after some time as happened to the original one in the pictures.

So I designed a replacement cap similar in dimensions and style to the original one but optimized for easier 3D printing. I printed in PETG since it is a bit flexible and friendly to food items. Technically the design should work with other materials as well though.

The cap is designed so that it only requires support for the inner stub as long as you print with a layer height of 0.2mm or smaller. Additionally, I printed with 5 perimeters and 10 solid top and bottom layers for additional strength. For convenience, I also attached a 3mf file with the support enforcer cylinder already in place.

Removing the old cap from the cap holder is tricky since it is strongly snapped in place. As such I designed a small removal tool, consisting of a small flexible ring ramp that you put around the original cap's waist with the thin part pointing downwards like in the picture. Then you put the larger and stronger ring from the tool below the cap holder and press firmly on the cap against the ring positioned on a hard surface. This should allow the original cap to snap out from the holder. The material used for the tool is likely not particularly relevant but again I used PETG.

After removing the old cap you put the cap holder over the new cap and firmly press it down until it snaps into the cap's waist. You might need to hammer the ring down a bit since the holder ring is extremely tight.