The ornamental design for a posttop mount for a floodlight fixture, as shown and described.
FIG. 1 is a perspective view from above of a post top mount for a floodlight fixture, the fixture and the post being shown in phantom lines;
FIG. 2 is a perspective view from below of the post top mount for the floodlight fixture of FIG. 1;
FIG. 3 is a front elevation of the post top mount for the floodlight fixture of FIG. 1, a rear, right-side and left-side elevations being identical images;
FIG. 4 a 45°-from-front elevation of the post top mount for the floodlight fixture of FIG. 1; and,
FIG. 5 is a bottom plan view of the post top mount for the floodlight fixture of FIG. 1.
The phantom lines are provided for illustrative purposes only and form no part of the claimed design.