I claim the ornamental design for the floor grate insert assembly, as shown and described.
FIG. 1 is a perspective view from above;
FIG. 2 is a perspective view from below;
FIG. 3 is a top view;
FIG. 4 is an end view;
FIG. 5 is a bottom view;
FIG. 6 is a side view; and,
FIG. 7 is a perspective view exploded, thereof.
The floor grate insert assembly is shown with symbolic breaks in its length in FIGS. 1-3 and 5-7. The appearance of any portion of the article between the break lines forms no part of the claim.