I stumbled upon this today and it may be useful for anyone looking for an internal duct for their rig:
Blue:
http://www.svc.com/ovcokitbl.html
Silver:
http://www.svc.com/ovcokitsi.html
Blue:
http://www.svc.com/ovcokitbl.html
Silver:
http://www.svc.com/ovcokitsi.html