The ornamental design for a convex-sided spa, as shown and described.
FIG. 1 is a perspective view of a convex-sided spa showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a side view thereof, the opposite side view being a mirror image thereof; and,
FIG. 4 is a front view thereof, the rear view being a mirror image thereof.
The convex-sided spa is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.
The broken lines in FIGS. 1 and 2 are included to show unclaimed subject matter only and form no part of the claimed design.