[plasman]

you could use epoxy (i use radiator epoxy, PC Fahrenheit - works great)like people said above, i have had great luck with it, and with a twist it comes off clean but holds strong.

Also you could try some very thick metal wire that bends easily and wrap it around the heater core, shroud,fan combo. Then put it thru the holes of the fan or shroud and twist like twist-ties. not sure if this will work in your setup, but its a suggestion.