Cordierite-mullite kiln shelf. 26" (660 mm) diameter, 3/4" (19 mm) thick. This is the recommended size for 10-sided top-loading kilns like Euclid's R-195 or R-280.
Buy 10 of any 1 kiln furniture item & pay 10% less.
The low-expansion property that allows this shelf to withstand repetitive kiln firings also applies to lower temperature applications. Kiln shelves make excellent pizza stones!