Hello Najary,
The cavity is in the fuselage. The fuselage is molded concave from the rear of the belly pan allowing the pipe to sit tecessed for another 6"-8". If you look at the picture of my Majestic on the ZN site you will see in effect the same thing, just not as long. I cannot say exactly how long a pipe it will hold but I would estimate about 240mm longer pipe length than the Synergy. The two piece winged version will probably be somewhat different as the belly pan will probably simply be extended fully farther to the rear.

Mark