Food grade Silicone O-Ring's work great for various 1/2 inch NPT fittings and are available in several formats:
Thick wall - 11/16 inch ID x 1 and 3/16 inch OD
Thin wall - 13/16 inch ID x 1 1/16 inch OD
Dash 211 - 13/16 inch ID X 1 1/16 inch OD
Thick wall - 1 1/4 inch ID X 1 3/4 inch OD
Dash 217 - 1 3/16 inch ID X 1 7/16 inch OD (heating element o-ring)