For fabbing CAI or WAI or brake cooling ducts take a look at dryer exhaust ducting. It's available in 3" and 4" diameters, perhaps other diameters too. It's flexible and all metal so it won't melt if it comes close to exhaust components.
That is egg-zactly what I used for my HAI, along with black plumbing PVC to route it into the hole that used to lead to the resonator box :
GasSavers_RoadWarrior 
cfg83 
Threaded Mode
