The ornamental design for a spillresistantcupinsert, as shown and described.
FIG. 1 is a front, perspective view of the spill resistant cup insert of the invention shown in place within the interior of an associated cup, the cup being shown in dotted lines as not forming a part of the present invention;
FIG. 2 is a top view of the spill resistant cup insert and cup of FIG. 1; and,
FIG. 3 is cross-sectional view thereof, taken in the direction of line 3-3 on FIG. 1, the opposite side cross-sectional view being a mirror image thereof.