I am, at the moment, cooling my dwc buckets by cool air from outside. I don't know where you live, but if you get temps below 18°C (65F), you should consider trying to vent some outside air directly over the buckets. I have placed some styrofoam directly on top of my buckets (made holes so only plants poke out), so my box is basically split into an upper part where the plants grow and the lower part where the buckets and roots are. It keeps temperature quite well, so it's very easy to keep cool. If you can't use outside air, you could get a small portable AC unit (again I don't know where you live, but where I live one can get a used unit quite cheap (70 USD maybe). and use it to keep temps at the lower part to aout 18°C.