I had trouble finding a replacement for an old ceilng lamp cup (don't exactly know if this is the right name for it) after the original one got broken, and so I printed myself one...
I don't know if this design will be of any use to anyone else, but since it solved a problem for me, perhaps it will help someone else as well.
UPDATE: Added a couple of pictures of the cup in its final placement!
I printed it in 0.2mm layers on my Arduino Materia 101, in white PLA.
Assembly is very simple, slide the small cone on the wire, then slide the cup, and make the electric connections. Once done, push cone and cup towards the ceilng until the cup rests flat on the ceiling, and tighten the grub screw slightly (don't over-tighten as you might damage the rubber coating of the wire).
The small cone that helps hold the cup on the wire requires a 3mm grub screw, and is designed to fit on a wire that is 6mm in diameter.
Please note that electric connections involve a certain amount of risk, and should only be done by professionals, and in any case always after powering of the mains switch!!!