I keep several box turtles. Try building your enclosure out of landscape timbers. You can make the enclosure any length you want but keep it less that 8 feet wide. By keeping the width under 8 feet you can easily use a sheet of covered plywood or corrugated roofing to make a sheltered area on one end. Also by keeping your enclosure narrow you can come up with a barrier a material to keep the predators out (chicken wire) and let sunlight in. Don't forget a pool of some sort and plenty of compost piles, the box turtles will love you for those feature. The compost will also provide protection from sunlight and predators. If you had a pair, such a set up could result in more.