The ornamental design for an undermountstoragestructure, as shown and described.
FIG. 1 is a top front right isometric view of an undermount storage structure.
FIG. 2 is a bottom front right isometric view thereof.
FIG. 3 is a front view thereof.
FIG. 4 is a rear view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a right side view thereof.
FIG. 7 is a top view thereof.
FIG. 8 is a bottom view thereof; and,
FIG. 9 is another top front right isometric view illustrating optional and exemplary environments.
Features shown as evenly-sized broken lines form no part of the claimed design. In FIG. 9, features shown as long-short broken lines illustrate an optional and exemplary desk environment that forms no part of the claimed design and an optional and exemplary drawer environment that forms no part of the claimed design.