The ornamental design for a hands-free umbrella harness, as shown and described.
FIG. 1 is a front view of a hands-free umbrella harness showing my new design;
FIG. 2 is a back view thereof;
FIG. 3 is a side view thereof;
FIG. 4 is an opposite side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
In FIGS. 2, 3, 4 and 6, the broken lines showing of environmental structure for purposes of illustrating the hands-free umbrella harness in a condition of use. The broken lines and the structure they depict form no part of the claimed design.