The ornamental design for a hair accessory stretchband, as shown and described.
FIG. 1 is a top plan view of a hair accessory stretch band showing my new design;
FIG. 2 is a perspective view thereof;
FIG. 3 is another top plan view thereof, the bottom being a mirror image;
FIG. 4 is a left side elevational view thereof, the right side being a mirror image; and,
FIG. 5 is a an end elevational view thereof, the other end being a mirror image.
The broken line representation of opposed combs in FIG. 1, and the broken line representation of the ends of the hair accessory stretch band that attach to the combs in FIGS. 1-5, are for illustrative purposes only and form no part of the claimed design.