RACO Square Flat Electrical Box Cover- the preferred flat box cover with 1/2 inch center knockout is used to complete the assembly for electrical boxes. The flat covers are designed for use with 4 in. square electrical boxes. The blank junction box covers are engineered with angled mounting slots for the easiest installation where you don't even have to remove the box screws. Saving you valuable time! The covers are built from steel and made to last.