The ornamental design for a stackablestorageelement, as shown and described.
FIG. 1 is a front and side perspective view of a first embodiment of a
stackable storage element showing my new design, it being understood that
the perspective view from the opposite side is a mirror image thereof;
FIG. 2 is a front elevational view of the embodiment of FIG. 1;
FIG. 3 is a view similar to FIG. 1, but showing a second embodiment
thereof;
FIG. 4 is a front elevational view of the embodiment of FIG. 3; and,
FIG. 5 is a side elevational view of FIG. 4 from the left, the side
elevational view from the right being a mirror image thereof.