A wall-mounted enclosure assembly comprising knockouts formed through one or more enclosure walls is provided with a layer of metallic or polymeric flame-resistant material affixed to the exterior or interior surface of the one or more enclosure walls to block the knockout's perforations, which would otherwise remain open. This seals the unused knockouts and prevents hot molten materials or flames from escaping the enclosure through the knockouts' perforations in the event of a destructive equipment malfunction within the enclosure. Unused knockouts remain visible through the tape, and consequently the tape does not prevent installers from easily locating and removing selected knockouts.