Hi Freddy,
I have a neck rest that I use on my Carbent and used on my ti-Aero before. For me, I have to get it at just the right height that it comes in under the helmets outer shell but its okay to me if it rests against the softer plastic of the rear adjustment. I ride with a Limar helmet but I have also used a Giro without any problems.
The difference might also be in the design of the headrest. Mine is only about 2" in diameter, just a 1: piece of pvc pipe attached to a piece of aluminum that attaches it to the seat. The PVC pipe is then covered with a piece of foam pipe insulation.
If you feel you need the neck rest and that one doesn't work, try altering that one or seee if you can't get another one. You can see mine here.
http://image71.webshots.com/71/6/76/...3wEqMye_fs.jpg