Rabbits getting HOT - How to keep them cool?

the rabbits are becoming hot and breathing more harder. I have got the buns in the shade and i put frozen bottles of water in the hutches and the rabbits tends to lick them .... But is there anything else i can do.

there outdoor buns ?

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You could try spraying water on their ears. (They cool done by sending the blood up to their ears to cool down.)
Also putting a large towel/teatowel (depends the size of the hutch/enclosure) up that is soaked. This is so they're in complete shad and that the wind that blows in will be cool