FIG. 1 is a top, front, right side perspective view of a corner insert for a packaging tray showing my new design; FIG. 2 is a front elevational view thereof; FIG. 3 is a left side elevational view thereof, the right side being a mirror image of FIG. 3; FIG. 4 is a rear elevational view thereof; FIG. 5 is a top plan view thereof; and, FIG. 6 is a bottom plan view thereof. The dotted lines are for environment only and are not part of the claimed design.