I claim the ornamental design for an elastic headband with zipper, as shown and described.
FIG. 1 is a front perspective view of an elastic headband with zipper showing my new design. The broken lines in the drawing are for illustrative purposes only and form no part of the claimed design;
FIG. 2 is a front elevational view;
FIG. 3 is a rear elevational view;
FIG. 4 is a right side elevational view;
FIG. 5 is a left side elevational view;
FIG. 6 is a top plan view;
FIG. 7 is a front perspective view of an elastic headband with zipper in an opened position;