Electric Fence Insulator. It can be fastened with a nylon zip tie or with some iron wire or a nail.
This is an open-source design. FreeCAD file is provided.
Feel free to modify this thing at your convenience.
Good design is hard work, donations are welcome.
Technical
PLA does the job. Maybe it's better to print this thing with ABS if you are in a ver hot region.
Lay it flat on the side to print it so no support is needed