Check out info on resin casting. You have a mold, mix the resin with a catalyst. Whatever is in the mold gets embedded in plastic. - This link doesn't include info, but it's a reminder to go check out more info on resin casting
Knead-a-Mold, from Townsend Atelier for DIY silicone rubber molds that set in less than 10 minutes. The final mold can be used in the oven (for polymer clay) and is food safe as well! Use to make soap?