It hurts me to say this because it is not MADE IN USA or CANADA, but, HF and Princess Auto, in Canada, sell an air hammer mounted in a "C" frame called a planishing hammer. I picked on up off Craigslist for $50 and it hurts me to admit that it works real well. There is a plastic plug on the back of the frame and this is where the frame should be loaded with sand. I used spent sand from our sand-blast-cabinet for fill. It is necessary to top-it-up after a time because the sand settles. If the frame is kept full, it works better because of the increase in mass and it is so much quieter in use.
OK, sheetmetal forming was my first trade, so I might have a little advantage here, but this thingee is really easy to use. Wear ear protection and start to work the metal in the centre of the patch. Do not make the patch any larger than is necessary because you will making extra time consuming work for yourself. It is not necessary to use anything thicker than 18 gauge because the thicker steel gets to more difficult to work. Some persons like to start pounding out the shape with a round face hammer and a sand bag and then make the curve smooth with the planishing hammer. I have done it both ways and both worked very well. Time: About an hour for both sides and the cost of material is less then $20. Check the fit often by putting it against the panel that the part will replace. Do not cut out the rusted area until just before the new piece goes on. YouTube is your friend, now, There are lots of good videos on the how-to-weld-it part. If you are not comfortable with welding, there are some really good epoxies available for the auto repair industry that are as strong as any weld on light gauge metal.
I built a set for a local guy's L and it took about an hour of my time. Sometimes the metal will work easier if it is heated with a propane torch now and then, I never went to red from the heat, and for Goodness Sake, do not cool the metal in any liquid, it will become hard as, well, very hard. A slow cool at ambient temperature is best.
http://www.harborfreight.com/media/cata ... _14637.jpghttp://www.eastwood.com/media/catalog/p ... 5/p767.jpgI have a selection of Eastwood hammers and this sandbag and they work really well. A pair of tough leather gloves make the job easier to hang onto and possibly safer.