Hi,
YES - we too have had fost on the inside of the coop/hutch. It did concern me, so I tried a duvet over the coop but that just ended up so damp its in the bin. I then tried a thermal blanket, the ones you get in a first aid kit- that froze itself. Plus it was so flimsy- it ripped very easily.
My five girls are inside a rabbit hutch, especailly bought for the winter months which is a lot more portable than their summer house. This is in our wooden shed having moved them from the metal shed. So nothing has really worked. Short of bringing the hutch into the house, I can't really think of what more can be done.......
Thankfully they are now huddling together having finally mastered the ramp up to the sleeping quarters. I have been lifting them up and down for the past few weeks. Bless them, they wait for me to offer my hand, they then climb on and get placed on the perch.