Evidently drainage pipe is heavy, and therefore difficult to move, set in place, and support—so it doesn't fall through the floor into the garage. Also, someone would have to cut through the steel to fabricate it to fit the space.
Evidently steel drainage pipe is hard to cut through.
In short, Sam and Dave told me I couldn't have my half-pipe, but as a consolation offered to design and build a fireplace surround covered with corrugated steel that would gesture toward the pipe dream.
The consolation feature was also a pain to build, though, so early on in its fabrication, Dave lovingly dubbed it "Mike's fucking half-pipe."
Here is how you make one.