In the linked Fusion360 tutorial I walk you through modeling a insert and cavity mold designed for making a silicon part. It uses some skills and techniques from my parametric grommet video, so I recommend watching that as well. Making simple custom molds for silicon parts is a great approach to creating small batches of finished products or prototypes. This is a great use of 3D printers. Included is the Fusion 360 file and step file and sample 3MF file.