The ornamental design for a foot warmer for open-toed casts, as shown and described.
FIG. 1 is a perspective view of a foot warmer for open-toed casts showing my new design in use;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof,
FIG. 5 is a right side view thereof; and,
FIG. 6 is a bottom view thereof.
The broken line showing of the human figure and casted foot is included for the purposes of illustrating environmental structure and form no part of the claimed design.