The ornamental design for a stackablestoragedevice, as shown and described.
FIG. 1 is a perspective view of a stackable storage device of the present invention;
FIG. 2 is a left side elevational view of the stackable storage device;
FIG. 3 is a right side elevational view of the stackable storage device;
FIG. 4 is a front elevational view of the stackable storage device;
FIG. 5 is a back elevational view of the stackable storage device;
FIG. 6 is a top plan view of the stackable storage device; and,
FIG. 7 is a bottom plan view of the stackable storage device.
The broken line showing of a sponge and a plurality of bath oil beads disposed upon the trays in FIG. 1 is for illustrative purposes only and forms no part of the claimed invention.