Child resistant outlet cover/plug (Or Dust Cover). Plugs into most any American outlet, holds tight and has beveled outer edge to make it difficult for children to pull out of outlet.

I needed one to keep dust and metal grindings out from an outlet on my workbench, strangely, couldn't find one to download so I made one.

I recommend standard PLA or a good Flex, Glow is neat too but DO NOT USE CONDUCTIVE, Magnetic or carbon fiber as these can sometime conduct as well.