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It's tricky as the long distance touring market prefers items which are fixed to prevent theft. They also tend to want to charge both a phone/gps on the handlebar and a device inside the rear pannier, hence the original bottle mount idea.
However you only need an M5 hole - the fork mudguard mount for example? It's also possible to put under a stem, as the tab is long enough to pass a long cable tie through and around.
What about a holes in the case to run zip ties through or a quarter-turn mount like Garmin? Could just glue one on and then it's easy on/off bars to swap bikes.