[morzh]

Nail polish might damage the button if it is a clear plastic (not backelite) due to some acetone being the solvent in it (unless it is not).
The film it forms will not last long if you keep pressing it: it is not a glue and will crack, and the cap will fly off again.

I would suggest a threadlocker, like Loctite.
You could also use shims.

The advantage of threadlocker it will hold things in place, but it is not a glue and so if you ever need this cap off, it will come off, even if not without effort. With glue it might take too much effort and then you might damage the cap.