Could something be mounted under the rear rack, under the top box? Just thinking that once you find/ buy/ build/ refashion something that holds the tools you need, it could be secured to the underside of the rear rack. It would have to be fairly thin to avoid hiding the tail light, however.
Only something very narrow as it would obscure the brake light.
AH, sorry I misunderstood that you specifically wanted to mount something externally, I thought you were looking for options to better organize your tools inside the bags.
No problem vanislejay. I just wanted to clarify. It doesn't hurt to have multiple options pictured for those in the future who stumble across such threads.