Bitmaps can be used as height fields for a 3D terrain. Typically, the brighter the pixel value, the higher the terrain at the location mapped to the pixel. A 3D grid such as a terrain model can be output as cross-sections and created in any convenient material. The Processing app included with this post will do the whole process for you: open a bitmap, display it as a terrain model, and output cross-section profiles to a PDF file. The profiles include slot marks. Once you cut out the profiles and the slots, you can fit everything together into a model. The model can be used to construct a mold for the surface.
Let me explain this with pictures. First, here’s what the assembled model looks like: