What's the best way of making fencing to close off a map? I was thinking of some low wooden fences to close off some fields, but I don't know if a model should be used or that it can be done with solids in hammer.

Since my map is gonna be for DoD: Source, is it gonna make any difference?

1) You could eventually use models, but is one of the worsest choices. Use models for ambience and details, seal off map by world brushes.
2) Use everything you can imagine to make boders, so it won't be repeat too often (houses, bushes, tree lines, houses, walls, fences and so on.)