Many on here have used the heating grade foam with adhesive backing found at Home Depot/Lowes.
I just use that heating/duct shiny silver 'foil' tape also found in the plumbing/heating departments of those stores.
YES, it IS 'sloppy', looks like crap, and is impossible to get ALL around the whole shroud (unless you pull the whole assembly out of the car LOL), but it fills in the huge gaps from shroud warpage, and will do until I get a Mountune/Pro Alloy radiator, along with a new factory shroud (or a higher flowing, bigger aftermarket fan sans shroud).