Here's a double pen setup that I built for my water turtles. I keep my box turtles in a similar manner (though without the 50 gallon stock tank). I like to use landscaping timbers - they are very durable. The biggest challenge I have is keeping predators out. I dig the "open air" setup that Jeff posted, but I've got raccoons, opposum and skunks in the neighborhood.

Tim
Third Eye
Third Eye