Oops, yes, 3.5mm is the diameter. I was sleepy when I wrote that, and I knew it didn't seem right.

Good point about the orientation when inserting. I was thinking something like hex or torx or friction fit that would allow you to insert it without thinking about the orientation. But then that causes another problem: any visual markings on the knob aren't guaranteed to be in the right position. I'm not sure if there's a good way get the best of both worlds on that.