now cover it with a layer of my personal formula of oogoo. liquid silicone and talc, like the feet talc but this is pure with no odor, you can buy it at farmacies.
The diference with the corn starch oogoo is that this one is water proof and equally cheap.
This paste will make the knocker harder enough to make noise against the door
now for the knocker you can use any rope, i found one with a shape i liked
For the painting, first a layer of black paint, then a thin layer of bronze paint that don't fully covers the black paint.