I claim the ornamental design for a modular storage unit, as shown and described.
FIG. 1 is a front elevation view showing my new design;
FIG. 2 is a right side elevational view thereof.
FIG. 3 is a left side elevational view thereof; and,
FIG. 4 is a perspective view of an enlarged detail of the lower right front corner thereof, the lower and upper right front corners being identical, and the lower and upper left front corners being mirror images of the right front corners. The flat rectangular panel that should be visible in this detail has been omitted for clarity of illustration.
The broken lines form no part of the claimed design.