I have fitted these to a Multitech recently (having got Wapping to fit them to my TourX.
Used a thin stick to push the thin wires into the space between the shell and the lining and stored some of the left hand wiring behind the cheek pad.
Then used duck tape to secure the thick wire underneath the neck collar as you rightly point out there is no bracket to fix it to. Used a bud microphone (at the advice of Starcom) and attached the wires to the chin piece using thin strips of duck tape, having pushed as much slack as possible into the shell where the whisper kit slots into.
I purchased the lid at the Exel show and went straight to the Starcom stand to ask them to fit a headset for me. Unlike Autocom they refused (I have noticed GS Rich of this parish is always fitting headsets for customers). Could be the deciding factor if I change intercoms in the future ....