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It's a piece of bent copper sheet, so that you don't put the tip (which I thought would get hot when I made the model) on the plastic itself. You can either find a bit of sheet copper and use the drawing in the instructions to get the same thing as in the pictures, or if you can't find copper, you could just quickly draw up and print a small plastic part that would fit in its place :)
As I found out later, the base of the soldering tip stays cold enough that you can touch it, so having it touch the plastic should be fine.