I built a system based on an exhaust-ducted Zalman 7000 about 5 years ago. It has been in constant use ever since. The CPU is an undervolted 2.4GHz Northwood. Airflow is provided by a single Nexus 80mm fan running at about 7.5V. The air is pulled from underneath the heatsink through the fins, along the duct and out the rear of the case.
First I made a shroud from bits of scrap metal to seal the gaps. I used a hose clamp to hold it together.
Then I made a coil by wrapping wire around a bottle.
I wrapped the whole thing in duct tape to ensure an airtight seal from the heatsink to the rear case fan. It's actually very robust and will not move or distort even when pushed quite firmly.