Really struggling here in Kent. Mid 30’s outside, and the thermometer in our outdoor enclosure topped out at 54degrees today!!! They always have a little ferret paddling pool available, into which we put ice cubes but they just fish them all out and run off with them! We also have picnic freezer blocks on rotation (only £1 each, and a bit less faff/space than bottles of water). We have stone paving stones in the enclosure, and drape a white wet sheet over the mesh.
For the majority of the day though, they’re out of thier enclosure and in a smallish run on the grass under trees in the shade in the garden, which is way cooler.
Bought one of those cooling mats today, which work simply by pressure, no freezing required. So when they sit on it, it cools down. Built for cats and dogs, so not sure yet how effective it’ll be on light ferrets.