Can I use polymer clay to make a mold and then melt HDPE plastic in it in the oven? My goal is to be able to dig a fairly uncomplicated shape in a polymer clay block and then fill that mold with small HDPE plastic filaments. I'm hoping that the melted plastic would fill the mold with the help of gravity pushing it down or I can get it out to apply pressure a few times. Will the polymer clay melt? Do I have to bake it beforehand? If clay doesn't work, what other easily found and cheap substitute exists? Thank you.