HungryHunter, if your in range, I would definitely take Gary up and workshop with him.
Personally, I f I were to make a flat horn I would use dry heat (i.e. a heat gun) and heat the horn as hot as I dared without burning it, then press it down in a vise or book press using several "shims" as the horn flattens an extra shim gets added progressivley until I got it as flat as I was looking for.
Now, that said, I don't have a whole lot of experience in this department so that is just how I would go about doing it. :idunno: hope this helps.