Non dif portal frustum calculation

For the past few days ive been studying the code, trying to figure out how to make a portal thats not part of a DIF, like if i wanted a mirror that could be moved, a DTS, etc.

ive figured out most of it, what im stuck on now is the computeNewFrustum call the mirror handles which uses dif related stuff, and seems overly complex.

my question is, does anyone know how to calculate the frustum manually, since the call a mirror uses is specific to DIF's