I use 18 gage about .045 it is a bit on the thick side but if you strech it or grind/file it you have more to work with. I do have some 19 gage. Tt is special order and not stocked by most sheet metal shops. It is mild or low carbon. 20 gage would be stocked and is easy to form. It is on the light side and will warp more. I would try the 18 gage. I have made many fender patches for the 47-55 trucks. What area are you patching?