We claim the ornamental design for a masking plug, as shown and described.
FIG. 1 is a perspective view of a masking plug according to an embodiment of the invention.
FIG. 2 is a first side view of the masking plug shown in FIG. 1.
FIG. 3 is a second side view of the masking plug shown in FIG. 1.
FIG. 4 is a third side view of the masking plug shown in FIG. 1.
FIG. 5 is a fourth side view of the masking plug shown in FIG. 1.
FIG. 6 is a bottom side view of the masking plug shown in FIG. 1; and,
FIG. 7 is a top side view of the masking plug shown in FIG. 1.
The present application is directed to the design illustrated in the drawings. The elements shown in broken lines are included for the purpose of illustrating environment and form no part of the claimed design. The broken lines having unequal line segments illustrate the boundary of the claim.