UnityFixNegativeBoxCollider.cs

    //
    // If the effective scale of a BoxCollider ends up making the .size negative,
    // such as when a prefab is scaled (-1,+1,+1), then Unity complains loudly:
    //
    //   /!\  BoxColliders does not support negative scale or size.
    //
    // This works around the problem by looking at the signs of the lossy scale,
    // and flipping the same signs in the box collider's size vector.  Since
    // the box collider is always aligned with the local matrix, this has no
    // effect on the collider except to quiet the console warnings.
    //
    public static void FixNegativeBoxCollider(BoxCollider box)
    {
        Vector3 lossy = box.transform.lossyScale;
        Vector3 flip = new Vector3(
            Mathf.Sign(lossy.x),
            Mathf.Sign(lossy.y),
            Mathf.Sign(lossy.z));
        Vector3 sign = new Vector3(
            Mathf.Sign(box.size.x),
            Mathf.Sign(box.size.y),
            Mathf.Sign(box.size.z));
        if (flip != sign)
        {
            Undo.RecordObject(box, "Fix Negative Box Collider");
            box.size = Vector3.Scale(box.size, flip);
        }
    }