My boys live in a hutch in my shed (year round).
In winter their hutch is stuffed full with hay (literally to the top), they have four snugglesafe heatpads between the two of them (they have one heat pad under each hide, and then two other spread elsewhere in the hutch).
On the hutch there is a thermal hutch cover, two blankets, and then a normal hutch cover. I then throw a duvet over all of it come midwinter. These measures work very well usually in keeping them warm.
Dont use any kind of fleece bedding inside the hutch as it won’t dry properly