I've got a strand of lights that have the "intermediate" E17 (Edison thread, 17mm) base. I don't need light bulbs in all of the sockets, but I don't want to leave open sockets laying around since there is line voltage in there. This is a handy little plug which will screw into the E17 socket and fill it up.
Protip: Don't print this with conductive filament.
Printed with 2 shells and 20% infill as it doesn't need to be particularly strong. PLA would work fine unless maybe this was to be used outside, in which case ABS or PETG would probably be a good idea. Layer height does not have to be too fine as the E17 thread has a pitch of about 2.8mm