The ornamental design for a paintcontainerinsert, as shown and described.
FIG. 1 is a perspective view of a paint container insert showing my new design;
FIG. 2 is a bottom plan view thereof; and,
FIG. 3 is a perspective view, showing the claimed design in use.
The broken line showings of a paint container in FIG. 3 are for the purpose of illustrating the environment in which the design is associated and form no part of the claimed design.