I've used light 1 - 1.2mm? expanded mesh with about a 5mm opening spaced off the radiator about 10mm. If you're radiator is alloy you could use stainless or aluminium. I've used the fluted style not the flat and mounted 180 degrees to what this pic shows. With the lean back angle of the radiator, it is only the thickness of the metal facing the direction of flow.
