The ornamental design for a foldable lunchboxliner, as shown and described.
FIG. 1 is a front elevation view of a foldable lunch box liner showing our new design;
FIG. 2 is a rear elevation view, the dashed lines are used to show the fold area located on the front of our foldable lunch box liner and form no part of the claimed design;
FIG. 3 is a left elevation view the opposite side being the same;
FIG. 4 is a top elevation view the opposite side being the same;
FIG. 5 is an isometric view of a foldable lunch box liner in a flat configuration; and,
FIG. 6 is an isometric view of a foldable lunch box liner in a folded configuration showing our new design. The broken lines representing an exemplary lunch box are for illustrative purposes only and form no part of the claimed design.