Some experts recommend a minimum of 4 feet by 8 feet for a box turtle pen, especially if you have multiple turtles or it is a full time home. If space is an issue and you only have one or two turtles, a smaller pen will suffice, but try to keep it at least 4 feet by 4 feet. In the wild, box turtles tend to roam over fairly large distances and will be stressed if cramped.
You will want to place your turtle pen in a sunny location - preferably where there is some sun most of the day, especially morning and early afternoon sun. Don't locate it in a heavily shaded location. Don't forget to provide areas of shade within the pen, however.
Solid sides are preferred by many owners, as some turtles will vigorously try to get through a wire fence, whereas if they cannot see beyond the walls they won't spend as much time trying to get out. Untreated wood or cement blocks are good choices. Heavy guage wire has also been used by some owners, but keep in mind that turtles may be able to climb a wire fence so you will need an overhang into the enclosure or even a cover (more on this later) to prevent escapes. At the end of this page I have listed some sites with photos and instructions for different pens.
Preventing Escape by Digging
Box turtles are good diggers, so the sides of the cage should be sunk into the ground. In addition, concrete paving stones placed around the inside perimeter of the enclosure flush with the ground will help discourage digging. Wire mesh can be also be laid flat a few inches under the soil extending from the walls well into the enclosure (use a fairly heavy wire for this to prevent turtles from cutting themselves on fine wire like chicken wire if they do dig).
The height of a turtle pen should be at least twice the length of your longest turtle. For box turtles, 18-24 inches should be plenty.
Covering the Pen
A cover can be made of a wood frame with wire mesh. A cover will help keep climbing turtles in, and more importantly, ptredators out. Keep in ming that wandering pets and wildlife can pose a threat to your turtles.
Furnishing the Pen
Hides: half logs, plant pots on their side (dug into the dirt a bit), or wood boxes (even small plastic igloo type doghouses). Have at least one hide per turtle, possibly more.
Water: a shallow pan of water (e.g. a saucer from a large plant pot) can be sunk into the ground. If you sink it into a gravel area it won't get muddy as fast. It must be easy for the box turtles to get in and out of the water pan.
Burrowing Spot: dig up an area and mix the soil with leaf litter, grass clippings (pesticide free!) shredded bark, or bark chunks to make a nice loose mix that turtles can easily burrow into.
Plants: plant the pen with non-toxic plants. Try food items like collard greens, kale, parsley, strawberries, raspberries, clover, alfalfa, etc.
Other Items: sticks, small logs, and flat stones give the turtles something to explore.
Sprinklers: if possible a sprinkler system is an ideal way to give the turtles a regular misting regularly. If not, remember to put a sprinkler on your turtle pen daily.
If you can't build an outdoor pen you should still strive to give your turtle time outdoors. A large plastic pond liner or even a kids pool can make a good outdoor playpen. Use cypress bark and soil in the bottom and add some hides and a shallow pan of water. Use potted plants for shade (and snacking).
Ornate Box Turtle Modifications
Ornate box turtles come from more arid areas and have a strong instinct to dig. They need lots of loose, deep soil for digging (and good safeguards against escape). Their pens should be planted with items such as prairie grasses, wildflowers, sagebrush and scrub oak.