Couldn't you use some sort of PVC pipning joint?
Something like:
http://www.usplastic.com/catalog/pro...Socket+Fitting
or
http://www.usplastic.com/catalog/pro...Socket+Fitting
are probably readily available in your local hardware store, are easy to cut down with a hacksaw, and are probably a lot cheaper than the pics I sent which are (in my opinion) illustrative only.
[EDIT:Below]
Also if you're only using an 80mm fan, you could use that duct you linked to and build a shroud type thing on the radiator end. :shrug: Upto you.