For casting a few items you can use wood, however eventually it will char. For something as fine as a medallion I might use plaster of Paris. It is relatively cheap and easy to work with. make you a pattern, coat it with release agent (grease or oil) and make several molds so you can pour several at once. I have had some luck with this process. Sometimes you can reuse the mold by just tapping out the casting , but sometimes the mold breaks. That is why I recommend making several before the pour. Good luck with your project. Show us some results if possible.