This perfectly seals up an unused light socket on this string light set.

To the best of my knowledge this string light set uses E26 bulb sizes.  I ended up creating an E27 size cap because E26/27 they seem to be pretty interchangeable sizes.  Why did I choose E27?  Well you have to create these threads manually in Fusion 360, and I could only find proper dimensions for E27 to be able to create this myself.  But it fits perfectly!

Printed with 2 walls, 3 bottom layers and 3 top layers, with 15% grid infill, and 0.2 layer height.  I used PETG since it will be outdoors, and I printed in black to match the black string light socket.