Widows like structures over top of them. That way they can build a nice retreat (either tube or bell), and can make their catch net, sticky dropping threads and everything else that goes into a classic and well-built Latro web. I prefer putting a thin layer of sand/dirt on the very bottom for aesthetic reasons, and I like easy access into the container. Here are a couple of examples of containers I like, these are not mine:
(Image courtesy of Pulk, on arachnoboards.com)
(Image courtesy of hauser, on arachnoboards.com)
These cages pictured above are not ideal setups, but they will help get you started. To get a better idea how to build your cage, you need to check out and understand the structure and function of a widow's web. You can find that here: web structure
And for further reading and insight on widow webs and design, check out my presentation: Environmentally-dictated web structure