Broan-NuTone® 885BL Wall Cap to provide reliable ventilation. Features a black enamel finish and is designed for use with 3 in. or 4 in. round ducts. Includes a built-in back-draft damper, and protective bird screen to prevent small pests from entering duct.
here's a link to the website that sells them for $40 https://broan-nutone.com/en-us/accessory/885bl