Definitely an angled thin sheet of aluminum that starts on the out side lip if possible and directed to the radiator core opening edge.
A little bit of finesse and patience will be needed to cut and trim to fit the many variations in that area. Do it first with a card board template to make the bends and trimming, then copy it onto a sheet of 3003 aluminum.
Cuts air fantastically and keeps outside cool air going where you'll need it most.
Wish I had photos of mine, but I don't.