The ornamental design for a fitness weightdestabilizer, as shown and described.
FIG. 1 is a perspective view of a fitness weight destabilizer;
FIG. 2 is a right end view thereof, relative to FIG. 1.
FIG. 3 is a left end thereof.
FIG. 4 is a front view thereof, the rear view being a mirror image thereof.
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines illustrate portions of the article that forms no part of the claimed design.
The claimed design is shown with symbolic breaks in three components' length, indicating the appearance of any portion of the article between the break lines forms no part of the claimed design.