The factory used lead, but the ability to use lead is a dying art form. Get all the info you can for using lead properly ect.. and then you might think about alumalead filler as opposed to plastic filler. The lead is the the best for the edges of thingsect, but I have seen plastic or the other options used as wel. too. Pick out the one that you feel the most comfortable with and stick to it.
You might even try to find some books by the experts on the topics to see what they say.