An accessory for a charcoal grill, particularly a kettle-type grill, that is designed to lay on the surface of the grill's charcoal grate and to partially block convective air flow in the grill while confining charcoal to a small portion of the charcoal grate. The accessory device increases the efficiency of the grill by about 50% as defined by the amount of charcoal required to cook food on the grill. Advantageously, the device keeps lit charcoal from contacting the inside wall of the grill so that no heat is lost by conduction through the wall of the grill and modifies the flow of air in the grill so that the heated air is focused on the food.