I recently rebuilt a headset for a head tracker mouse for a paraplegic chap.
This instructable contains a bit of information that might help anyone else trying to do the same.
The unit that this headset works with is the PRC Headmaster, Prentke HM-2P. Its a fairly old design but it is based around ultrasonic transducers and time of flight calculations.
Step 1: Print the headset
3D printed using PET+ (tough and flexible).
STLs on Thingiverse or attachedMk4V1A3.stl
Step 2: Electronics
Quite simple electronics
16mm SQ40R ultrasonic transducers
Nokia headset wire to add a flexible bit at the end (turns out they use 5 cores).
PSF 100A pressure switch (lowest pressure version)
Bits of ribbon cable to join it all
Connections numbered in the diagram
Step 3: Assembly
Mostly push fit.
Add a bit of heat shrink to protect the headphone wire.
Velcro and foam to add some padding.
Sugru to protect the exposed electronics.