I've talk to the factory this morning, and a girl told me that there was nobody who would help me at that moment. She took a note about the issue, and they will call me tomorrow.
Anyway, I've continued the work on the plane: started to cover the wings and ailerons... with some problems. I will post pics tomorrow, with a more advanced work.
Yes Funtana, the ply rings conform the support for the cowl. The cowl must be mounted in order to be flush with tyhe fuselage. As I had the same question made to a friend of mine, he advised me to add 6 ply blocks around the firewall rings to mount the screws. Four are sugested, but two more are recommended at both sides of the silencer (in the case that you use a 2 stroke).

Tomorrow, pics and the factory response (I spect!).