What are they called?

[mrlmd1]

And why would you want to spend $10-20 for an authentic motorcycle "puck" with or without someone's emblem on it if you can do it for almost free and accomplish the same thing? To each his own I guess. Those things are thicker than a switchplate, electrical box cover, or flattened beer can and take up more room in a bag.
The only advantage of a plastic or rubber puck over a metal one would be to have something that maybe the kickstand wouldn't slide off of, but that's theoretical, I've never heard it happen to anyone. You could always bend one edge up a little to form a lip of sorts , or turn the switchplate so that the little lip faces upward if you were worried about that.