The two bikes that I have with such a fender mount also have a hole for mounting a lamp on teh fork crown, but if I did not have a hole for mounting a lamp and if I did not want to drill a hole for that purpose, I would consider fabricating a piece of Aluminum bar stock in an L shape to give me a new bracket. In other words, I would try to make something like this.
https://www.homedepot.com/b/Building...s/N-5yc1vZc8hr
For a while I put a lamp down on the bottom of the fork using one of the fender mounts, but I did not like that position because I thought it created too many shadows at night. But you might consider that option, see photo.
A friend of mine bought one of those gizmos (not sure what it is called) that attach to the end of a skewer so that you can mount a handlebar mounted headlamp down on the skewer. I told him he would not like it because of the shadows, but he proved me wrong - he only uses a light during daytime, never rides at night.