304 Stainless Steel "Slim" Washers. These washers are 1/16 inch thick and come in several sizes:
1 1/2 inch ID - 2 1/4 inch OD (will fit kettle elements)
1 1/8 inch ID - 1 5/8 in OD
1 inch ID - 1 1/2 inch OD
7/8 inch ID - 1 3/8 inch OD
Simply select the size you need by Interior Diameter (ID)