IF A CARAVAN CAMPED IN A VALLEY AND IT WAS SURROUNDED BY THE TRAPPINGS OF THE CATTLE, IT IS PERMISSIBLE TO MOVE OBJECTS WITHIN IT; PROVIDED [THE TRAPPINGS] CONSTITUTE A FENCE TEN HAND BREADTHS IN HEIGHT AND THE GAPS DO NOT EXCEED THE BUILT-UP PARTS
This is echoed an expanded by a beratia (mishnaic era text which did not make it into the written document):
Come and hear: If a caravan camped in a valley and it was surrounded by camels, saddles, saddle-cushions, saddlebags, reeds or stalks [it is permitted to] move objects within it, provided there is no more than the space of one camel between any two camels, that of one saddle between any two saddles, and that of one saddle-cushion between any two saddle-cushions!
This kind of ad hoc barrier might be limited to caravan (which is perhaps defined as simply more than 2 people) but it gives an idea of the attempt to make the law achievable for people in different situations. A caravan might have difficulty constructing an actual fence (although a rope fence with stakes is also permissible). Having that kind of space is important – animals have to be fed and cared for, even on Shabbat. A workable space would have to be defined.
If needed, the camels themselves, along with their saddles, might be considered a fence!