For RenderToTexture/Camera based portal rendering, the basics seems simple enough.

However, with a free camera, most of the time it is going to be looking at such portals at an angle:

Now a regular near clipping plane will not always work here, it will either intersect with the wall the portal is sitting on, or possibly with objects in front of the wall.

The desired near clipping plane would be aligned like the portal, producing a view volume more like this:

or this in 3D:

So here is my question:

How does one construct or "truncate" a view/projection matrix to achieve such an off-camera-normal (near) clipping plane?