First, determine the size of the missing knockout hole and subtract 3/8" to figure out what size knockout plug you'll need. For example, if you have a 7/8" hole, you'll need a 1/2" knockout plug ...

Google “ ¾ -inch knockout plug,” and loads of possibilities pop up. Knockouts are the places where an electrical box has been punched through to allow for wiring, so connections for outlets ...