A simple spinner swivel
Spinners are an attractive alternative to a flag. They're more visible, particularly when you're stopped. Some poeple say they create less drag too.
Even though spinners have a rotary swivel, they still have a tendency to tangle up and wrap around the pole as the wind direction swings.
I made a simple swinging swivel for the top of my flag pole. I picked a pole with a metal ferrule. If your pole has no metal, you can still fix a washer to it by drilling a small hole through the pole, pushing through a bit of copper wire and soldering the washer to that. Then dab epoxy around it to strengthen it.