I have no suggestion for how to mount it. But, I have a suggestion for something you probably had not thought of yet.
I have bought some older Louis Garneau (spell?) handlebags over the years and found that they lacked adequate stiffening and would sag badly.
Example:
And that was with a handlebar bag that the manufacturer had already added some stiffening to. Your camera bag lacks that.
I added two pieces of Aluminum bar stock, 1 1/2 X 1/18 inch to the bracket that are bent to go underneath to add support to the bottom. I initially tried two pieces that were 3/4 X 1/8 that was inadequate but the wider pieces seem to work well. They are attached at the mount and extend underneath. I do not have a great photo, but you can see them here on teh same bag that is on a different bike.
Whatever you do, the lack of stiffness of the bag will probably require some retrofitting.