Custom frustum culling with scissoring

Hi, I've waded through the forums but could not anything regarding my problem. I was wondering how one would go about creating such a custom frustum culling as can be seen on the attached screenshot. It seems that this custom frustum has a shape of a cube inside camera's frustum. Also note that the objects seem to be scissored on frustum planes (the sphere on the right).

I'm not sure that oblique frustum is the right way to go.