HRTF is based on size, shape AND ABSORBTION characteristics of WATER BASED HUMAN HEAD. Our heads ABSORBS (not reflects) most ALL AUDIO FREQUENCIES.
In other words, water based flesh absorbs sound, reflecting almost nothing off its surface. My Sonic Studios designed GUY and LiteGUY HRTF baffle remain the only (100% acoustically correct) available today.
and most practical www.sonicstudios.com/liteguy.htm
Guy/LiteGUY baffles are not easy or cheap to make so maybe only practical for the serious amateur or professional needing true HRTF performance, but these cost many thousands less than (far less effective/accurate) well-known synthetic dummy head alternatives.
So using cotton for absorption qualities was a step in the right direction, but has far too little absorption
to work even OK over the very active and hard Styrofoam material. MOST all synthetic materials do not absorb audio frequencies effectively, and in fact, while some foams/rubbers do absorb some
portions of the audio band, they are quite ACTIVE in making significant changes to the frequency amplitude/phase OF THE ACOUSTIC PRESSURE signatures needed for HRTF baffle fidelity. In other words, they COLOR the sound (making EFFECTS) in noticeable manner to be a liability to making consistently good sounding recordings.
The only really cheap and good sounding HRTF (type) baffle for those on a budget remains cotton fabric covered
~head shape/size hard-stuffed (fiberfill/cotton batten/wool) pillow or even a cotton-cord-bound bath towel
or similar constructed baffle works more like what's needed for consistent good sounding results. Baffles made of these fibers can be ~2/3's head size and still work well. Best to have mic spacing/baffle width ~7-8" even if head height is shortened to as much as 50% for having a compact easier to transport baffle.
In summary, all those other materials do not absorb enough, and are active in altering the sound in audible ways not HRTF
. No matter how good those other (synthetic material) dummy head baffles may look to the eye, they are producing significant anomalies in HRTF acoustic terms.