The ornamental design for a base for carseat, as shown and described.
FIG. 1 is a top rear perspective view of the base for car seat in a closed configuration, showing our new design;
FIG. 2 is a right side view of FIG. 1;
FIG. 3 is a front view of FIG. 1;
FIG. 4 is a top view of FIG. 1;
FIG. 5 is a rear view of FIG. 1;
FIG. 6 is a bottom view of FIG. 1;
FIG. 7 is a left side view of FIG. 1;
FIG. 8 is a top rear perspective view of the base for car seat in an open configuration, showing our new design;
FIG. 9 is a right side view of FIG. 8;
FIG. 10 is a front view of FIG. 8;
FIG. 11 is a top view of FIG. 8;
FIG. 12 is a rear view of FIG. 8;
FIG. 13 is a bottom view of FIG. 8; and,
FIG. 14 is a left side view of FIG. 8.
Broken lines in the drawings denote features of the article that are included for the purpose of illustrating context, but form no part of the claimed design.