The 60318-4 is desinged to simulate the effects of the middle and inner ear. It is another piece of the puzzle which is also being used in GRAS' Ear and Cheek Simulator or Head Acoustics' HMS.
Actually the response of the coupler does also include a bit of the ear canal, right in front of the ear drum. So the 3/4 canal is probably fine. The canal of the latest anthropometric pinna by GRAS (KB5000) may even be a bit shorter.
I am trying to give the outter contact surface roughly the same softness as the inner one so that the ear pads can sit on an even surface. Silicone would be the best. Maybe I can find some thin cell foam that is not too hard. Most are open cell and may cause leakage, though.