Don't bother. 80mm fans don't have the pressure to be worthwhile on radiators. Most hit near zero flow at the resistances put up by radiators. I tried ducting a few 80mm fans onto a HUGE heatercore. There was literally twice the radiator surface area as there was fan area and they still barely moved any air through. Natural convection would have been only slightly less effective.
It was really quiet though. Nevermind that a pair of 5 volted 120mm L1As blow 10 times the air
