I found a "PET bottle cup" at a 100-yen shop. A cup that can be used like a water bottle just by attaching it to a PET bottle. The price is 108 yen (tax included).
Easy to install! Simply cover the PET bottle cap with the attachment and push it all the way in. All you have to do is snap the cup into the attachment.
Hygienic because you don't put your mouth directly on the bottle. It is also recommended for small children who are not good at drinking directly from PET bottles.
You can use it like a water bottle!
It is compatible with various sizes and can be used with 500 ml, 350 ml, and 280 ml PET bottles. When I attached it to a slim PET bottle as a trial, it did not come off, although there was some wobbling compared to a bottle with a general shape.
A 100-yen shop "PET bottle cup" that allows you to use a PET bottle like a water bottle just by putting it on. If you have one, it will be useful outdoors!