I got 3 cages from Amazon and rigged them to fit together (Atuvian is the brand). It works pretty well, so far. It has a plastic lined bottom, but we added duct tape a bit up the side to keep the wood from damp rotting and to conjoin the bottoms so we could get a nice deep substrate for (s)him to dig in. We decided to use a twin bed frame as the base for even support.
Here is a link to mine a few years ago