How to instantiate an object with a set rotation in one axis, but the other two axes inherited from the parent?

I'm trying to spawn an object with the x axis rotated to 90, but the y and z axes having the same rotation of the spawner. The spawner runs through random items (in this case, it's for decorative flags) and then spawns it.

The reason I need it to be 90 on the x axis is because of how cloths are orientated by default. To have the object up a wall, I need to rotate it. If I instantiate it with transform.rotation, it assumes I also want x, so it gets messy as it can get stuck in walls and the like.

Here's my code; however, it ignores the spawner's y and z and flags always face the same direction!

 #pragma strict
 
 var partsPrefab : Transform[];
 
 function Start () 
 {
     var z = transform.rotation.z;
     var y = transform.rotation.y;
     var rot = Quaternion.Euler(Vector3(90, y, z));
     Instantiate(partsPrefab[Random.Range(0,partsPrefab.Length)], transform.position, rot);
     Destroy(gameObject);
 }
 
 function Update () {
 
 }

It's my first time working with angles like this, though, so I'm a bit lost. Is there any way I can have x = 90, but y and z be transform.rotation?

Any help would be appreciated!