The ornamental design for a base for a sunshade, as shown and described.
FIG. 1 is a perspective view of a base for a sunshade showing my design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof. The broken lines in FIG. 7 are included for the purpose of illustrating portions of the base for a sunshade that form no part of the claimed design.