The ornamental design for a snap-on necktie knot cover, as shown and described.
FIG. 1 is a perspective view of the snap-on necktie knot cover shown in use;
FIG. 2 is a rear elevation view of the snap-on necktie knot cover;
FIG. 3 is a top plan view of the snap-on necktie knot cover;
FIG. 4 is a side elevation view of the rear of the snap-on necktie knot cover;
FIG. 5 is a front elevation view of the snap-on necktie knot cover;
FIG. 6 is a bottom plan view of the snap-on necktie knot cover; and,
FIG. 7 is a side elevation view of the front of the snap-on necktie knot cover.
The broken lines shown in the drawings illustrate portions of the snap-on necktie knot cover that form no part of the claimed design. The partial human figure shown in broken lines in FIG. 1 represents how the claimed design may appear when in use and forms no part of the claimed design.