Gabi, I've never had a plane with absolutely no structure in the back end that would accept a brace and glue. Both sleeves can be extended all the way to the back of the servo and anchored there with braces. If you don't want to cut a hole in the covering then melt a hole as indicated above and let the red sleeve extend about 1/8" outside the covering and glue it there with a little epoxy or CA. Or, use one of the plastic pushrod exits with a hole to accept the red sleeve. However long you make the sleeve, make the inner tube a little bit longer and anchor the Kevlar at one end of the inner tube.

I know that this is not the case with your plane but this pull-pull method works very well with planes where the control surface is not directly in line with the servo.

Ken