You don't have to make it out of putty, use stylene, wkma7six made a turbocharger from scratch on his ITR,
http://www.automotiveforums.com/vbulletin/t44126.html
There is also a thread for building intercoolers,
http://www.automotiveforums.com/vbulletin/t59570.html
Hope this helps yah