The best option is to get a rock or ceramic/terracotta item. Put it in the freezer for an hour and then put it in the cage. Even 30 minutes in the freezer is good. Then it'll be something cool for them if they need it.
If you get more than one item per cage then you can swap them as needed through the day too
We find keeping our curtains closed helps stop the heat from getting in with the sun as an extra measure. Its actually all we need to do in warm weather fortunately