Steel Knockout Hole Plug for Indoor and Outdoor 1/2″
https://nsiindustries.com/product/steel-knockout-hole-plug-for-indoor-and-outdoor-1-2/
Use Bridgeport 1/2″ Steel Knockout Hole Plug for Indoor and Outdoor to cap unused knockout holes in a box or enclosure. Operators can secure the knockout hole by pressing or hammering the plug into place.