I customized the entry of Gloomsen (https://www.printables.com/de/model/211006) a little bit:

• thin out the walls, for faster printing
• add an edge to the cover and thicken it a little bit, to make it easier to remove the cover from the socket
• size up the top cover by 2mm in diameter, for more coverage of the socket hole
• I uploaded my blend file so you may customize on your own.

Print with:

• no support
• no infill

These cover were made for German / European sockets. ATTENTION: the covers are not to childproof your sockets!