Well, you don't really need a "cage" per say. I use a tortoise table. It is pretty much just a table that has about a six inch (will vary depending on size of torts) wall all around the perimeter. I lined the "floor" of my tort table with laminate flooring for cleaning purposes. Then I use coconut fiber ( the kind you buy in the bricks and soak in the water.) for subtrate, peatmoss works well too. You need both heat sources and UVB/UVA bulbs. A large but shallow water dish, plenty of hides, basking spots, and if you have cats you should think about a screen top of some sort. I have also built an outdoor pen that I use during the day in the summer.
Here is a good expample with instructions.
Tortoise Table, Tortoise House, Tortoise enclosure, Tortoise Viviarium