I got the idea for this off Pinterest, but I do not know whom to credit for the idea. After I made one, I found it was limited up to about 18mm, so I have made another.
This holds up to 38mm
It takes only 10 seconds to change between them, but normally they will hang on a hook with the spring.
Not very even, is it? The one in the second picture shows the spring attached to a coach bolt driven into the stool, which is maple, but unfortunately itís too dark to make that out.