I found the Human Skull model while searching for bookmarks, then decided it needed to become one.  The model was scaled down and smoothed, then the bottom was cut to mount on a bookmark.

I printed the skull at 0.1 layer height and 15% infill to get the best resolution without waiting too long for the print to complete.

The gcode file is set to print supports (Snug) in all areas except the holes for the bookmark piece and the ear canals.  This further reduces the print time, and allowed for simple removal of the supports.

Your printer might print the bookmark piece too thick or thin to slide easily into the skull.  If this happens, use the scale function of your slicer to adjust the thickness of the bookmark and reprint just the bookmark piece.  Remember to scale only the thickness and not the whole piece.

After printing, carefully remove the supports and sand any rough areas.  Once you have a good fit between the two pieces, apply a small amount of glue to hold them together.

Thanks to Jimson for the original model.