I've sculpted a new model out of clay. Now I only had my hands and a butter knife as tools but I think I turned out well. Next step is to create the mold and cast it.
Here's the result! I let the silicone degas on it's own since I don't have a vacuum or pressure pump. I wasn't able to completely remove the air bubbles but I added a coating of silicone by hand. The surface is almost completely smooth.