The ornamental design of a self-sealing sink cap, as shown and described.
FIG. 1 is a top plan view of a self-sealing sink cap showing my new design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a front side elevational view thereof;
FIG. 4 is a rear elevational view thereof; and,
FIG. 5 is an enlarged bottom perspective view thereof, the release paper
being raised so that the broken line perforations being shown for the tear
off tab.