I've been toying with the same idea.
I would imagine they are mandrel bent and then heat treated. Don't know if heat would be required for the bend. Having done some (limited) bending of tubing and round stock in the past, I can tell you that it ain't as easy as it looks. It's tough to get the angles just right without building a jig, and you have to deal with some springback.
I'd bet it would be
TONS easier and cheaper to just buy some used ones and be done with it. Unless you like this kind of stuff
(I do)
I've been toying with making some adjustable bars for my 911, or possibly just adjustable lever arms and drop links that will work with the stock bars.
Have fun,
Jim