Restricting rotation

I've got a flying object that can rotate while flying forwards, using a float variable that is connected to the input.acceleration (phone tilt). I want to restrict the rotation to about 45 degrees or so, so that it doesn't start performing barrel rolls.

I've read up on it but what I've found so far hasn't helped me.

I tried:

         Vector3 eulerAngles = transform.eulerAngles;
         if (eulerAngles.z > 45 && eulerAngles.z <= 180)
             {
                 transform.Rotate (-z * speed2, 0, -x * speedxrotate * Time.deltaTime);
 
             } 
             else if (eulerAngles.z < 315)
             {
                 eulerAngles.z = 315;
                 transform.eulerAngles = eulerAngles;
             }

Unfortunately it doesn't work. The object is just permanently rotated.