Boeing installs the plug. Its an option based on the airlines layout - high or low density config. It is done at Everett before they deliver the airplane.
Well, Renton actually.
Ah, neither. The fuselage is built in Wichita by Spirit AeroSystems and shipped to Renton by rail as a basically complete structure - plug (or door, as specified by the aircraft buyer) already installed.
You guys! I believed you all one by one but now I don’t know what to think. Will there be another correction? Bloody Internet.