We claim the ornamental design for a monofin with flexible boot structures, as shown and described.
FIG. 1 is a top perspective view of a monofin with flexible boot structures, showing our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a front view thereof; and,
FIG. 7 is a back view thereof.
The dash-dash broken lines illustrate portions of the monofin with flexible boot structures. The dash-dot broken lines represent the bounds of the claim. The broken lines form no part of the claimed design.