Where I live, is literally right next to a nature park...kinda. anyways...I have tons of wild animals...cats, dogs, racoons, opossums, deer, foxes, squirrels
. Anyways...The walls on my enclosure are about 12 inches high, but I built a wooden frame that attatches to the back of the enclosure, with chicken wire on top, so that it acts as a lid. It also locks to the main frame, so it can't be removed by coons or the like. Only people or wild monkeys, or any other animal with opposable thumbs and the combanation can unlock the cages...lol
There really isn't a hard part about keeping them. Obtaining them can be hard, and expensive, but the hardest part in it's self would be the size enclosure they need. but you've got it all figured out with the 30 or 40breeder.
Make sure you research all the differnt types of american box turtles. They are all the same species, but are all different subspecies, and all have different habits. ie.
Florida Box Turtle-Very hard to find, likes marshy habitats, found naturally in the everglades. smallest subspecies.
Eastern Box Turtle-illegal to own in it's native habitat without special liscenses or such. Lives in mountain forests. Eats mainly berries.
3-toed box turtle-a dessert species. eats more insect matter than other box turtles. grass hoppers, crickets, etc. but eats alot of veggies too.
western/ornate box-I'm not sure