[sc_rufctr]

Like Larry suggested I would go with poured concrete. (with steel reinforcing bars)
You could make an inner and outer mold from wood. It's easier than it sounds.

I would make the top out of steel plate. Secured to the sides with brackets.
You could have it cut out of two parts instead of one to make it less expensive.

Just one obvious thing... It becomes a very permanent structure. A pain to remove if things change.