Model a 3D Compression Mold

I need a 3D model (.stl and .step formats) for a compression mold.

I have an .stl file of the part to be molded. A compression mold allows space for extra material to be pressed into a cavity. The design is more than simply creating a void in a mold box.

Please see the example starting at 3:11 here: https://www.youtube.com/watch?v=25PmqM24HEk&pp=ygUQY29tcHJlc3Npb24gbW9sZA%3D%3D