How to display a transparent fill in the palette?

Question

User interface

The task is to display a legend for a chart or map.
The colors are customizable by the user, so the fill color can be white, red... anything, including transparent (relevant for the map).
The legend displays squares of approximately 20x20 pixels filled with the appropriate color. It's like a color picker in a graph. editor.
In this case, a transparent square is obtained in the same color as the background - that is, white. And white in the legend occurs by itself.

Question: what is the best way to visually convey transparency without using labels? :-)

Answer 1

The famous black-and-gray (black-and-white) squares won't work?

Answer 2

You can also use light gray slanted lines.

Answer 3

Why display a transparent fill in the legend? After all, it is not visible on the map.

Answer 4

My wife suggested a white square crossed out diagonally - it seems to me that a user who is not familiar with graphic editors will be more accessible.