Hello Jean,
First, mount the pipe to the fuselage. Center it as much as possible in front. Then use a pen to mark the inside of the tail cone and remove material so it is flush. If it is a little loose, add some shims on top, below, left side and right side instead of adding a fiberglass ring. That way, you will have a little bit of air flowing.
As to your 2nd question. The first flashes had them, it appears to be optional on the EVO and, if I remember properly, the EVO user manual doesn't mention it. I don't have them on mine and have plenty of flights without any issue. Yet, this appears to be something CARF feels is necessary as, for example, the new Bolt has them... In my opinion, this isn't necessary when using a pipe with double tubes as the outside tube doesn't get very hot anyway.
Arnaud