First off, I am aware that there is a similar thread to this, but I wanted to go over things or share some new ideas
1. A frozen waterbottle near their bed.
I don't recommend using this tip for wild chewers. (From the plastic) But this is a good way to keep them cool while they sleep. Don't put it too close to them as it might scare them.
2. Ice cubes in their waterbottle!
I do this all the time when its hot. However, its hard to fit the icecubes in, so I break them up and put them in. It should stay frozen for a couple of hours. Make sure no icecubes are too big, if they are it shouldn't be to much of a problem but its not great to swallow them, since they are so small.
3. A cool-off zone
I use like a ceramic toy or bowl, you can use clean rocks/stones for this but I don't, anyway, to get the ceramic cool, I put it in the freezer for a couple of hours, then put it in the cage and they can sit/lay on the cold ceramic or stone. If you use stone, its great to cut-down their nails!
4. Refreshing waterbottle everyday
Some people do this all the time anyway, but I tend to only do it once or twice a week. Anyway, when the weather is hot, I empty and refill everyday to keep it fresh and cold.
5. A cool sand bath
For their sand bath, I put It in the fridge for a few hours. Meaning when they roll around, its nice and cool. Too make sure its not too cold, leave it out in the shade for about 15-30 minutes, but that's mainly if you feel its really cold.
That's all I have, feel free to share more ideas in the comments.