The ornamental design for a spabase, as shown and described.
FIG. 1 is a top, right, front perspective view of the spa base;
FIG. 2 is a top view of the spa base;
FIG. 3 is a bottom view of the spa base;
FIG. 4 is a right side view of the spa base;
FIG. 5 is a left side view of the spa base;
FIG. 6 is a rear view of the spa base;
FIG. 7 is a front view of the spa base; and,
FIG. 8 is a top, right, front perspective view of the spa base in its environment.
The broken lines showing a spa chair and foot rest are directed towards environmental subject matter, all other broken lines show portions of the spa base than form no part of the claimed design.