A temporary cover for covering a central opening in an electrical box cover plate is disclosed. The cover has a front wall, a side wall forming a rim around the periphery of the front wall, a connecting mechanism on the rim for releasably connecting the temporary cover to the box cover plate in a position in which the front wall of the temporary cover is spaced forward from the central opening in the box cover plate, and a release mechanism on the front wall for removing the temporary cover from the box cover plate. The rim of the temporary cover is configured to function as a guide for guiding a cutting tool to cut an opening in the wall member prior to installing an electrical device in the electrical box. A cover assembly including such a temporary cover is also disclosed.