Help! How do I keep my rats cool in the summer?

I have 2 5 week old rats and they are not aloud inside. Their cages go in the shed but i take their cages outside alot. There is a heat wave of about 40 degrees and the shed feels like a boiler room but its just as hot outside. Iv begged and begged to keep them inside until it gets cooler but its not gonna happen. Iv tried cool packs, ice, wet towels over the cage, and they dont like swimming yet but they are still so hot? Any suggestions would be greatly appreciated. I know i seem a bit paranoid but my cousin's rat died of heatstroke yesturday so i got a scare RIP Cleo