One of my pairs have a little Trixie Natura house, but they are newly together and one sleeps it in, one on top of it on a blanket at the moment. When the girl, Sweetie was in there with Alfie they both slept on top of it.
The other pair are in a kennel/run and have a cardboard box and blanket at the moment, but they have a bale of hay in the run part which they burrow in and spread all over then make little nests in at night. I plan to get a small dog kennel to go in the 'indoors' of the kennel for them to use over winter if they want and may get the same for the other two as I want something bigger than the Trixie house really, maybe 1 1/2- 2 times the size.