Create Negative Volume STL Mold

I have an original part in STL format and need a clean “inside-out” version of it. The goal is to generate a negative-volume shell suitable for mold making, but I only need low-detail fidelity—fine surface textures can be ignored as long as the overall geometry and draft lines stay intact.

Here’s the workflow I have in mind:
1. Convert the supplied STL into its exact inverse so the interior space matches the original external form.
2. Close and thicken the shell so it can be printed or machined without leaks or self-intersections.
3. Produce a STL file to use in a CFD model software

Please return:
• The negative model in STL (and STEP if convenient).
• A meshed version in STL ready for simulation, with a quick note on cell count and quality metrics.

No intricate texture capture is required—speed and simplicity trump sub-millimetre accuracy for this job. If you normally use tools like MeshLab, Blender, ANSYS Meshing, or snappyHexMesh, feel free; just keep the file names clear and the topology clean.