Are you asking for a half mold? Anyway if you have a solid and a you put it in a larger cube... Reverse the faces and you have a "negative object". But perhaps you want half molds--which it seems are needed to print. Then you can put a plane down the middle of the object and the cube and intersect faces. Depending on your shape this can be simple or it can require more planning and knowledge.
For printing the shape has to be solid and a clean, solid original shape should lend itself to being a mold. I'm just saying it shouldn't be a trick or require special tools. The shape is the mold itself, you just have to show where the mold material is around it.