Reverse moulding

Hi, hope someone can help me out here. I have the stl files for several gear shift knobs and would like to use these to make 3D printed moulds so that they can be re-cast accurately numerous times.
I have no experience of Blender or Meshmixer, they fry my brain, and I’ve never used a 3D printer so what I’m really asking is:

is there anyone who can either help me with a step by step workflow or perhaps even create the files for me if it’s not too arduous a task for the knowledgeable?

many thanks.

The general idea is to model a rectangular prism, import your STL, then subtract the STL from the rectangular prism. How you do that will depend on what software you are using.

In FreeCAD there are a few steps. Here’s a thread where someone was trying to subtract a screw from a solid, which is logically equivalent to what you are trying to do:

Thanks Michael!

TinkerCad would also be able to handle this and it’s the simplest 3d modeler that I’m aware of. There are plenty of tutorials you can google to get you started.

Here is an example.