Peggy Hall offers a formal Notice of Discrimination (or Notice of Violation) you can use when harrassed about not wearing a mask.  The purpose of the document is:

(1) To educate businesses and employees that their discrimination against you is prohibited by federal law.
(2) To document any violations of your civil rights.
(3) To notify the violator of your civil rights and that criminal charges and an official complaint may be filed against them.
(4) To constitute a visible response to your distressing interaction with violators of your rights.

Click here for the Notice of Discrimination and information on how to use it.