So, the backpack method sounds smart, but does it always work that fast? My cream was room temp and it still took forever, even with a good shake. Maybe the jar needs to be full enough to slam the cream around properly.
Put the jar in a backpack and go for a walk, it takes half the time and zero arm pain. Idk why everyone says ten minutes of shaking, mine always comes together in like five, maybe my cream is warmer? The key is you stop once it separates, not after some set time. You just end up with buttermilk splashing around for no reason.