Hi Gregorio, I hope this helps you
What to you mean by a metal structure? A bed frame, canopy or headboard?
You could use industrial pipes and join them to the size that you want. Alternative if you can't weld you could find a local handyman and get him to build you one.
Are you talking about the one the mattress lies on? I think it would be cheaper to buy one. You want one with the bar down the middle to keep mattress from sagging.