I always try to do this (use the map limits as part of the enclosure), and found it only works sometimes.
Did it on Isla Matanceros with a small enclosure for 20 Struthiomimus and 5 Edmontosaurus, no problem.
Tried again, a bigger one on Isla Muerta for a carnivores enclosure, and it went well at first, my 3 Ceratosauruses had a nice viewing to the sea...but when I added raptors they inmediately escaped by going briefly through outside the buildable area.
So I thought "it only works with big dinos" before I was proven wrong in Isla Sorna, when one of my Giganotosauruses decided to go trekking in the mountain before going back to her paddock.