You could try draping it with wet towels, the evaporation of the water has a cooling effect. This is still used today to cool down cask ales, for example where they are on gravity in the bar. Otherwise you could get some freezer blocks, attach them to the keg with bungees, and surround with bubble wrap. Many years ago Boots used to sell a system whereby a nunber of freezer blocks were placed in a harness around the plastic barrel and covered with a sort of duvet tailored to a barrel shape , it did work.If you are into DIY it should be no problem, alas, I am not.