I use black construction paper for the areas of my mirrors that are not covered by a monitor. It’s easy to cut into the appropriate shape and I can punch holes through it for things like camera lenses.
A piece of construction paper the size of your acrylic/glass can have a cutout the size of your monitor pretty easily. I don’t bother with tape or other adhesive to stick it to the glass. I start with a piece the size of the glass and it doesn’t go anywhere.