Generally, if you have space enought, the tendency is to creat individual or double boxes with a corridor in the middle in order to allow the horse make his own exercises wherever he want in a natural way. Additionally, this modules are much easier to clean than other options, as the horses remain most of the time in the outside part so the inside is not so dirty. Many horse lovers have the stable open during the day to allow the horse enter and go out in his rythm and only close the animals in the night to protect them from cold climates and maitain a more glossy coat.