Can you put an empty baking sheet in the oven?
Another option is to place your casserole in the oven on the top shelf and a larger empty baking dish or cookie sheet on the shelf just below it. If any food from the casserole boils over and out of the dish, it will fall into the larger dish or pan below.
Rather than using the middle racks, place your baking sheet directly on the floor of the oven. This will “provide the hottest, most even and direct heat possible.”
Can you put food directly on oven rack?
Presumably the reason you’re cooking directly on the oven rack is to let the fat drip down. If so, do yourself a favour and get yourself a roasting rack. It’s an inexpensive piece of equipment that you place in a baking pan or on a cookie sheet; the food cooks on the rack and the pan/sheet catches all the drippings.