I used to use a cheap, unglazed, ceramic 14x 14 tile from Home Depot. Had about an inch or so to spare on each side. It didn't break in over 15k miles, and we had a rear kitchen. But even if it did break, they are readily available at any big box store, for under a dollar.
I didn't put it directly over the flame. It was on the metal plate over the flames. Worked great to distribute heat. I used my oven almost every day. Bought a Sunbeam oven thermometer at WalMart for about $7.
Don't overthink it.
Edit... our oven was a Magic Chef