FIG. 1 is a top perspective view of a bowl for spa foot bath showing my new design; FIG. 2 is a bottom perspective view thereof; FIG. 3 is a top plan view thereof; FIG. 4 is a rear elevational view thereof; FIG. 5 is a left side elevational view thereof, the right side and front elevational views being identical; and, FIG. 6 is a bottom plan view thereof. The broken lines shown in the drawings represent unclaimed portions of the design and form no part thereof.