It's probably not really practical, the way you are imagining it. You can make the RF stronger in certain directions, e.g. horizontally. But a vertically-oriented antenna element will already do that, on its own. Do a search for "antenna pattern". To do anything more-specific, you would need to be able to either measure the RF field, or simulate it (e.g. with the free 4NEC2). Metallic objects will have effects (as will things like the height above the ground), but they might be much more difficult to predict than you might predict.
You have the in-ground fence, to make the device trigger. What else do you need, exactly?
Also, assume that we do not know how those systems work, or what components are located where.