It seems to be comfortable when brushing your teeth. I found a product called "Removable cup with suction cup" at Daiso and bought it.
"Removable cup with suction cup" is a product that includes a polypropylene cup and a suction cup. By attaching it to the mirror of the washbasin sideways, the water inside the cup can be easily drained and it can be dried efficiently. If this is the case, germs will not grow easily, and it seems that you can always use it in a clean state.
There is an unevenness on the bottom of the cup to combine with the suction cup
If you attach a suction cup to the mirror,
Click into the unevenness of the bottom of the glass,
It's OK if you turn the pointed part of the cup down
The suction cups can be removed, so you can easily attach them anywhere. If you see it at the store, please pick it up. The price is, of course, 108 yen (tax included) per piece.