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The reason it isn't working is that there is only one solid in the file. Usually you would want two models (in Cura after setting extruders then select both then right click and click "Merge"). If you only have one model file, there must at least be two meshes in the file. To separate these parts you would need to manually select all the parts you want, separate them, then extrude them so they are solids. The easiest way is to duplicate the faces before separating them, then extruding them (otherwise you'd have to fill the holes in the main body). In other words, this requires highly skilled modeling work. I suggest finding someone to do it or paying someone or finding a model that is already suitable.