The ornamental design for a spa shell, as shown and described.
FIG. 1 is a top front perspective view of a spa shell, showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a side view thereof;
FIG. 4 is a side view thereof, opposite that shown in FIG. 3; and,
FIG. 5 is a view of the longer side, the opposite side view being identical.
The bottom of the article is plain and unornamented.
The broken line showing in FIGS. 1 and 3-5 is for illustrative purposes only and forms no part of the claimed design.