A tip I got off this forum, you can try to place a ceramic cup on its side inside the cage to make an excellent cooling hideout for your hamster. If you have two cups, you can place one in the fridge so that it is chilled, then swap them out when one grows warm. Your hamster will love a chilled cup.
If you have chinchilla sand, you can chill it in the fridge and place it in a tub for your hamster to roll around in.
Here's a DIY cooling pad idea you can try out:
https://m.youtube.com/watch?v=p552YzdB2lM
I would recommend you prepare ceramic, aluminium, stone surfaces for it to lie on.
What temperature are we talking about for your area? It has gotten up to 35 Celsius on average in my area, and I would definitely appreciate more tips on cooling off from everyone else as well!