Quaternions, relative clamped rotation, look rotations in spaceship movement

I've come up against my limited knowledge of Quaternions, and I've almost solved the problem, but not quite.

In the code below, the pitch and roll of a ship work exactly as expected, EXCEPT that when rotation a "direction" transform that determines the forward direction of the ship, the rotations are then additive, so rotating the "direction" of travel 90 degrees yields a 180 degree rotation in the ship.

The ship must be constrained to one direction of travel, as if it were in a tunnel. the tunnels may change direction, but the ship's rotation must remain relative to the tunnel's main direction.

Here's the code below. It all works as I want, except for additive rotations between direction and final forward direction of the ship:

Thanks in advance!

 using UnityEngine;
 using System.Collections;
 
 public class QuatTest : MonoBehaviour {
 
     public Transform direction;
     public float roll;
     public float rollSpeed;
     public float pitch;
     public float rotationSpeed;
 
     protected Transform trans;
     public Transform container;
 
     protected Quaternion rollQuat = Quaternion.identity;
     protected Vector3 travelDir;
 
 
 
     void Start () 
     {
         trans = transform;
         container = trans.parent;
     }
     
 
     void Update () 
     {
         container.rotation = direction.rotation;
         travelDir = container.forward;
         Rotation();
 
         roll += rollSpeed;
         roll = Mathf.Repeat(roll, 360);
 
         Debug.DrawRay(trans.position, trans.forward * 10f, Color.green);
         Debug.DrawRay(trans.position, travelDir * 10f, Color.white);
         Debug.DrawRay(trans.position, trans.right * 10f, Color.red);
     }
 
     protected void Rotation()
     {
         //this line ALMOST acts as desired:
         // without LookRotation, AngleAxis yields completely wrong angles and ship rotates wildly
         // with LookRotation as is, rotation "direction" transform creates an ADDITIVE rotation between the two,
         // so rotating the "direction" transform 90 degrees yields a 180 degree rotation in the ship, BUT
         // the pitch and roll then behaves as expected on the resultant travelDir and trans.right vectors
         // i've tried replacing travelDir with trans.forward, and this yields the same result
         rollQuat = Quaternion.AngleAxis(roll, travelDir) * Quaternion.LookRotation(travelDir); 
 
         trans.rotation = Quaternion.Lerp(
             trans.rotation, 
             Quaternion.AngleAxis(pitch, trans.right) * Quaternion.LookRotation(travelDir) * rollQuat,
             Time.fixedDeltaTime * rotationSpeed);     
     }
 }