Mary acquired this bottle a long time ago and has used it forever, so it has Historic Connotations and cannot be discarded. Alas, the cardboard-and-plastic seal in the cap finally disintegrated; I replaced it with various plastic foams and sheets, none of which worked.
Finally, I found the cork sheet stash while looking for something else and cut out a disk:
It works perfectly, of course, because cork.
That was easy.