The ornamental design for a shower shelf, as shown and described.
FIG. 1 is a perspective view of the shower shelf;
FIG. 2 is another perspective view of the shower shelf of FIG. 1;
FIG. 3 is a left side view of the shower shelf of FIG. 1;
FIG. 4 is a front view of the shower shelf of FIG. 1;
FIG. 5 is a right side view of the shower shelf of FIG. 1;
FIG. 6 is a rear view of the shower shelf of FIG. 1;
FIG. 7 is a top view of the shower shelf of FIG. 1; and,
FIG. 8 is a bottom view of the shower shelf of FIG. 1.
The design consists of the feature of shape, configuration, pattern, and ornament of the shower shelf shown in solid lines in the drawings. The portions of the article shown in broken lines form no part of the design.