As long as there is a layer of material that has different density than air, Flood filling effect clicked on outside air should work. It will not leak into airways as long as neighborhood size is large enough.
Also, it is always an option to use ‘Grow from seeds’ or ‘Watershed’ effect. Create a segment and paint a couple of strokes inside the head, in multiple slices. Create another segment and paint outside in the air, in multiple slices. Then switch to ‘Grow from seeds’ or ‘Watershed’ effect and click initialize. Add more strokes where the segmentation results shown are not correct.
There are many segmentation tools. You have to spend some time with all of them to see what they can do (with slightly varying input parameters) and see which work the best for you.
If you can find similar data sets among sample data sets (in Slicer sample data or in TCIA database) or share anonymized data then I can try a couple of methods and let you know which of them seem the most promising.