You can put up an outside wall or something like that.I am sure if you look around here someone has fencing.I know if you buy Gravematter it has fences.I always try to put something up to cut of the point of vision beyond the boundaries of what I've done.Not only does it tend to look better but the farther you can see the more polygons are counted against you and it is a definite speed killer,meaning your frames per second are going to drop like a rock.
Doing outdoor areas in FPSC is a challenge.If you search the forums you can find some posts dealing with this.Good luck and welcome to the forums.
Not changing this sig until I get rich