Flynn sleeps on the bare wire of his crate bottom... no he's not an abused, neglected child - he likes it that way. The plastic tray the crate came with lasted 2 days and here's a list of stuff we've tried and Flynn has chewed into two inch square pieces or rolled up like a sleeping bag and slept on the bottom anyway:
Plastic floor tiles, piece of carpet, piece of vinyl flooring, towels, piece of old duvet cut to size (had great fun pulling the stuffing out), vetbed fleece (he whittled it into a realistic looking sheep pelt shape), ordinary fleece blanket, rubber backed rug put under the cage (managed to pull it through!) and stuff I'm sure I've forgotten about. Even when he's out, he prefers to sleep on the tiled kitchen floor (usually flat on his back with his legs in the air) than the carpeted from room.
I'm trying to get hold of a cow mat but I think that will go the way of all the others.