The keys things to remember when building a tortoise pen is: they can dig, and they can climb.
Most enclosures I have seen use plywood or other solid material for the sides (tortoises will constantly try to escape if they can see through the walls). The walls are buried into the ground at least a foot or so (more for large species) and usually have an inward-facing cap of some kind (especially at the corners) to prevent escapes. The height of the wall above ground should be at least 1.5 X the length of a mature tortoise.
Besides that, just provide a shelter, a shallow pool for soaking, and loose soil for digging and the tortoises should do well.