I have wondered if it would be possible to experiment for
how big a diameter and how long a resonator near the tailpipe exit
has to be by "fabbing up" two pvc pipe resonators.

I think the exhaust gas temperature has dropped to a level where pvc would hold up for a few hours but one could clamp a small piece of pvc to the tailpipe exit and do some test drives to see if it melts at full throttle up a steep hill.

My fuzzy memory seems to whisper that Larry Widmer in Texas claimed to have run a hopped up Honda with the back six feet of the exhaust made of "dimpled" pvc pipe that also acted as a muffler.