Better option is probably to lose the mount altogether. Looks like the bar mount is bolted on. Undo that and replace with a flattened pipe or piece of sheet metal
This is my thought - bolt is rounded off (possibly made of cheese) so will require a bit of work, but yes this is good thinking.
Best bet is probably attaching to rack strut like above, meaning a can slide something on. Some kind of sleeve over rack strut with an arm which then fixes to the bolt.
Better option is probably to lose the mount altogether. Looks like the bar mount is bolted on. Undo that and replace with a flattened pipe or piece of sheet metal