The lamp post in the photo is 5.9mm in diameter. The internal portion of the replacement knob is 5.6mm. The knob is press fit so that it sticks and stays in place. Once pressed on, it is very secure. You can remove it, but it is a bit tight the first time around.
The STL is included so that it can be resized or remixed as you need for your needs.
The author marked this model as their own original creation.