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I had good results with S3D doing this:
I lowered the model -1mm or -2mm Z height. (preview to see where you like it most)
2 layers of 95% infill raft, and checked "Disable Raft Base Layers". 2mm raft offset from part (again preview till it fills in the middle of the model). 0mm separation distance.

This made the skull with a 2 layer solid floor that is attached to the model. You can then trim the excess raft from the outer edges and have a solid flat floor skull model.