I know you need to be cognizant of the threat of predators on the ground as well as in the air when housing them outside. They will burrow underneath any fenced area that isn’t substantially at least a foot underground. Potential for escaping the fenced area has to be dealt with effectively. Groundhogs , feral cats , foxes, raccoons will all attack and kill turtles and feed on them. Hawks, buzzards, birds of prey also.