GRK is right. If you want to make it look good you have to spend a bit of time getting the joins just right.
However...
I recommend you think carefully about what you are doing. It is unlikely that, in reality (if there is such a thing) anybody would build a fence right across an island. Is that
really what you want or are you just being a bit lazy?
What about making the mission more fun by having certain [believable, predicatable and appropriate] consequences if the player chooses certain routes or goes to certain places?