What kind, size of box section, how big would the final structure be, is it for payment, how local do they have to be ? So many questions so little time.
Just a rectangle as in 4 pieces welded together, nothing on top filling the rectangle? I could do this for you no problem. Steel alone will cost about £50-60
I'd be asking for maybe £100 as a round number to do it?
Pm me if its of interest and we can talk more specific