On a cossie I would do all I could to help the water cooling, by not adding even more load to it, esp on a 3 door or 2wd, there's plenty of coolers that can go behind the fog aperture, even if they don't get full flow of air over the whole cooler.
Some small ones are found on carts, small motorbikes, dirt/pit bikes etc, and a very good price too.
tabetha