I assumed that there would be more ideas, but looks like there are none. So, I will throw out one more idea.
You can buy plates that are called Cantilever Brake Boosters that attach to the canti brake mounts, you could put a light on one of those plates. They are intended for bikes where the canti mounts flex too much. That said, I have no clue how much clearance you have below your bar bag.
The shipping time looks like it is shipped from asia.
https://www.amazon.com/Aluminium-Bic...dp/B08THVNXF8/
I bought one of those plates to use on another fork that I have, but have not used that plate yet.