i tried that with my piggles, but they actually avoided it until i'd wrapped it so much that there was no point in it being there.
i'd advisse general stuff, like making sure they have plenty of shade, and always have water. Tesco sell these great ice cube 'bags' that freeze the water to fit into drinks bottles. these ice blocks fit perfectly into pet drinking bottles, and it tends to keep the water cool for a long time.
hope that helps -c