TotalSegmentator segments real patient images, which look completely different than the scan of a 3D-printed object. In your 3D object scan, segmenting the “heart wall” is trivial, using thresholding. You can limit segmenting the ventricles inside the heart using “Wrap Solidify” effect (provided by SurfaceWrapSolidify extension), then splitting the ventricles using Scissors effect. Once you learned how to use the tools, the whole process should take just a couple of minutes.