I've never had much luck with chicken wire. It rusts after a few years for starters and becomes brittle. Not to mention its easy for turtles to climb. The key to keeping box turtles inside the pen is to trick them into thinking they're not contained. If a box turtle comes across and obstacle it will either climb it, dig under it or go around it. I use heavier wire fencing made up of large 2"x3" inch rectangles. The turtles can see through it very easily so they don't feel contained and don't bother trying to climb or dig under it. It's also green so that helps it disappear even further. When I first put them in the pen they would try to go through it instead of climb it. After a few days they learned their new boundaries. I have never seen a box turtle try to climb or dig under a fence when I used this approach. Just my 2 cents. 

