The ornamental design for a topcover for a spray gun, as shown and described.
FIG. 1 is a perspective view of a top cover for a spray gun showing our new design;
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;
FIG. 6 is a top view thereof; and,
FIG. 7 is another perspective view thereof in use.
The broken line showing of the spray gun in FIG. 7 illustrates the environment of the claimed design and form no part thereof. The remaining broken lines in the drawings illustrate portions of the top cover for a spray gun and form no part of the claimed design. Since the bottom view is not visible during normal use, the bottom view is omitted.