Bayern, I've posted another picture that shows the whole seat. I used a piece of plastic and turned it in my lathe to the same diameter as the rubber pads. I made a groove or a knurl (I don't know the proper English word for it) that fits snug in one of the slots in the rubber pad. When the seat is mounted, the spacer can't go anywhere. You could also use a pin or similar that fits the center hole in the pad, as long as it stops the spacer from sliding out sideways.
