I put down pallets,a layer of plastic sheet, covered with sheets of ply, or off cuts of 'tongue and groove' wooden flooring (We have a good supply of these for our arcs). Build up the ground around the bottom of the pallet to stop wind blowing under it. Then I scatter plenty of straw, so that they can make their nest, and still have enough to scatter a bit outside. If you don't they'll most likely spend the winter lying in wet bedding..not a pleasant thought.
You can save yourself a lot of money in bedding if you use my method. When we started we had no flooring and spent a more than ideal amount of money on straw. We had to, otherwise the poor creatures would have been miserable