How do I limit vcam depth movement (z) without using Confiner?

I have a 3d game project with a fixed camera perspective; (imagine 2.5d game or a beatem up game where you can move in the z-axis). When I move in the z-axis, using Cinemachine vcam with framing transposer, the camera follows. I want the camera to stop moving at a certain point when I move to close in on the screen (negative z).

The reason I don't want to use a Confiner for this is because from what I understand, the 3D colliders are limited in shape. What I mean by this is I am actually using the Confiner now but only for x and y axis confinement because I need ithe confiner to be an irregular shape and the Polygon Collider 2D does this but I think there's nothing like this in 3D.

So I was thinking maybe there are other ways to limit or confine the z-axis movement of vcam. Can someone guide me on how do I go about to achieve this? I'm thinking to still use my current Confiner but only x and y confinement and then some custom code to confine the z-axis but I don't know how I can achieve this.

 

How do I do that? I tried something like this:

Code (CSharp):

1. void Start()
2.     {
3.         _vcam = GetComponent<CinemachineVirtualCamera>();
4.         _framingTransposer = GetComponent<CinemachineFramingTransposer>();
5.     }
6.  
7.     // Update is called once per frame
8.     void Update()
9.     {
10.         if (_vcam.transform.position.z < -10f)
11.         {
12.             //_vcam.transform.position = new Vector3(_vcam.transform.position.x, _vcam.transform.position.y, -10f);
13.             _framingTransposer.m_CameraDistance = 10;
14.         }
15.     }

But it throws a null reference exception. Also, is it possible to just stop the vcam from moving?