Trying to generate a simple answer to that... Not sure which is shorter, saying how I might do it or pointing out what I don't like about the repair shown. I need to crash out.
Biggest thing was the overlapped metal and partial welds. And prying like heck on the package tray. And the complete lack of mention of any corrosion protection. Blatant disregard for proper dimensions. Theres no reason not to butt weld structural repairs and theres every reason to sufficiently protect the metal all the way through the repair. Fact is, if you found ten guys who "do side work", eight of them would probably be that sloppy. But all ten will take the money and say "fixed". Its tough to find good help, so if you aren't down to fix it yourself I suggest paying for a warranty at a good shop. I can't see charging more than five or six hours for that. If you want to DIY, a welder won't be all you need. But then again, a whole other world of "stuff you can fix now" would appear.
I need to go look at my own towers. I have 3 of these cars (2 wrecked) and I bet if any of them are rusty at all there its the one I drive. It seems my area is fairly kind to steel ...