Slim and sleek, a 12-inch stainless steel Line One Letter Plate designed by Atelier 522 is water-tight and has a spring-loaded hinge.
A Rope Mail Slot with a patterned edge is available in three finishes (lacquered brass as shown, unlacquered brass, and hand-antiqued brass) and is 11 inches long; $125 to $135 at Charleston Hardware.
In a satin nickel finish, a 10-inch Marcus Letter Plate is £22.50 from Ironmongery Direct.
Some people like labels. It is 10 inches long and available in four finishes, including Antique Brass, shown, for $69.95 from Signature Hardware.