Progress,  page 106
Here we are after pouring the concrete around the ductwork form.
I was able to reach down inside the ductwork and (using a Dremel tool with an abrasive cutting
blade) cut enough slits in the sheet metal to rip it all out of the hole.
That all worked well enough that I just kept adding sections of ductwork, building up the concrete
block around it, and pouring concrete until I got to the position I needed to be for a clear shot
through the roof.