I claim the ornamental design for a base for a toy, as shown and described.
FIG. 1 is an upper left front perspective view of a first embodiment of a base for a toy showing my new design, with the toy shown in dashed lines for illustrative purposes only and forms no part of the claimed design;
FIG. 2 is an upper left front perspective view thereof;
FIG. 3 is an upper right rear perspective view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a right side elevational view thereof, the left side being a mirror image;