The ornamental design for a cover for a navigationdevice, as shown and described.
FIG. 1 is a perspective right front view of a cover for a navigation device of the present invention, as viewed from below; and,
FIG. 2 is a perspective right rear view of a cover for a navigation device of the present invention, as viewed from below.
The ornamental design which is claimed is shown in solid lines in the drawings. The portions of the design shown in broken lines or dashed lines are included for the purpose of illustrating environment and form no part of the claimed design.