Best gap between fan & radiator?

There is a cool DIY fiberglass fan shroud project somewhere out there:
make a center mark on your radiator from the fan, take fan diameter and make a ring from out of something like cardboard 1" larger and exactly the distance from the radiator to the middle of your fan blade width. Eg. Sort of a 18 X 3.5" tall tube. Remove radiator, place wax paper over radiator sides or something that wont stick. Place form on radiator centered on mark and then drape fiberglass cloth over the entire radiator so it looks like a tent. Cut to size and stretch it a little so its taught over the form. Add resin and a few more cloth resin layers. let it set up. Now you can remove it off the radiator (wax paper) and youll have a rigid for fitted shroud. Just cut form out and paint flat black. there you go, a shroud. fan should be 1/2 out of shroud and have 1/2" clearance all around. OR you can make a sheet metal box with a fan circle cut out. It doesnt seal up to the radiator sides well without some sort of AL tape, but looks nice on a rod.