The post Zog is referring to is by #rotor, here:
http://forums.procooling.com/vbb/sho...+shroud+makers
But my whole point is that most likely 80mm will be enough without restricting flow. If you're using an 80mm fan, then an 80mm duct is certainly good enough. If the duct is restricting the air flow then so is the fan(note that 80mm fans do not push an area of air pi*80^2), and the fan is hindering performance. If you're not going to use the fan then there might be justification to use a larger duct, but - maybe not - your radiator can be quite restrictive (depending on many factors).
I also had another though of material you could use. You know when you had to vent your clothes drier? That metal tubing stuff comes with lots of fittings, again check your local hardware, im sure you can find something about the right size, and it will look way pro.