The ornamental design for a strapoverlay for an article of headwear, as shown and described.
FIG. 1 is a perspective view of a strap overlay for an article of headwear.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines within the solid line boundaries of the strap overlay depict the elements of an applied pattern that forms no part of the claimed design. Broken lines in FIGS. 1-7 adjacent to the shaded surfaces depict portions of the strap overlay for an article of headwear that are not included in the claim.