Both the cheapest and fire resistant is the clay. Archaeologists find clay tablets with inscriptions that survived even after Vesuvius eruption. Thereby you may  get  the clay, put it into 24 molds (each of ~10 x 5 cm  size), write down one seed word on the surface of the clay and bake the  molds at ~ 300 - 900^oC . First two hours the temperature should exceed 400^oC. In fact each type of clay requires its own heat condition so you need the preliminary test before doing the final attempt. The finished plates need to be  cool very slowly, during not less than 24 hrs, DYOR. Don’t forget to number every plate.