What is the correct way to write witch's hats as in the type used in sport to mark out areas?

I am not familiar with the term witch's hats, but it depends on how many
witches you envisage. If it's the hats of one witch, then it's witch's hats.
If it's the hats of several witches (which would seem the more likely), then
it should be written witches' hats.

It's explained in the apostrophe lesson here

Rgds