I'm not on a computer right now, so searching isn't as easy, but I've seen a couple posts over the years of people making that vent stack out of aluminum sheet (which is actually what I do now as well). If you use aluminum or other metal, just remember that the stack will be more conductive than the original plastic. I actually insulate mine and then build a cabinet around it.
While you're doing it, you may as well incorporate a couple of
12v fans in the stack. They will keep your refer much cooler in the summer. You can install a manual switch, or a thermostatic switch so the fans only run when the refer gets hot.