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I like something other than a square box if you are thinking of carpet over the metal. It might make it a challenge to cover a square.
This might sound funny but you could by a large metal bowl for like salads and stuff and just weld it in.
Here is an idea of what I'm talking about. Not necessarily SS but you get the general idea.
http://www.ikea.com/us/en/catalog/products/50057254/
You can get them in different diameters and you could even cut them down to the height that you wanted. Ok quit laughing...
Marc