This is a threaded plug (cap) for electrical junction boxes. There are both 1/2-inch and 3/4-inch versions as well as versions that include a hex thumb screw for easy tightening/loosening (which I highly recommend--who wants to carry around a huge flat-head screwdriver just for plugs?).

Note: I highly recommend using some dielectric grease on the threads to ensure water can't get into your electrical box.

Remember: In order to comply with the National Electrical Code (NEC) you'll have to print this in a flame-retardant filament and protect it from UV rays somehow (either by using UV-stable filament or painting with a UV-protective coating).

Printing:

Print @0.2mm layer height with three perimeters and it should turn out fine.  I used 15% infill but that doesn't really matter much (it's just a plug--it'll be fine).