We claim the ornamental design for an kettle grill accessory, as shown and described.
FIG. 1 is an environmental perspective view of the kettle grill accessory, the broken lines representing a kettle grill forming no part of the claimed design;
FIG. 2 is a top perspective view of the kettle grill accessory;
FIG. 3 is a bottom perspective view of the kettle grill accessory;
FIG. 4 is a first side view of the kettle grill accessory;
FIG. 5 is a top view of the kettle grill accessory;
FIG. 6 is a second side view of the kettle grill accessory;
FIG. 7 is a bottom view of the kettle grill accessory;
FIG. 8 is an end view of the kettle grill accessory; and,
FIG. 9 is a cross-sectional view of the kettle grill accessory taken at line 9-9 in FIG. 6.