G'day Scott!
How about this?
Make the lead slug. Give a thin coat of epoxy or similar glue. Wrap in Clingwrap. Spray with silver mirror and then some blue paint (from B*****gs and Supercheap).
The silver is $18 a can but should do lots of slugs.
When the slug hits the rocks, the cling wrap will "pit" but remain intact. The paint should stay stuck to it.
When the lure gets too battered put on more glue and another layer of cling wrap and paint.
Rob.