I did this, mounted a fender but I don’t remember exactly how I did it. It was for a friend and he lives 70 miles away. I posted photos of it here about 6 years ago so I don’t remember what to search for. I know I used 1/2 inch wide by 3/16 thick flat stock from the hardware store. I remember cutting a piece long enough to straddle the width of the fork and making round bends using a steel rod for a mandrel. The round bends have to fit the fork tubes precisely and be at the exact right distance apart. Don’t ask me how I did this but it was perfect. I think I attached the flat stock to the fork tubes by drilling holes through the rounded part of the flat stock and perhaps used sheet metal screws to keep it in place. I must have hung the fender with a nut and bolt to this cross piece using the tab that was on the metal fender. I used a black fender that is still attached using this method.
I found the link but I don’t know how to move it here. It is under “1953 Schwinn Black Panther”. It shows what I did to mount the fender and it was only 5 years ago, not 6 years ago. Anyway, it worked to mount and position an aftermarket fender as the original stainless ones were gone.