I like the idea that Truman suggested, but it does create some degree of concern for how you organize the rotation and the ability to serve in other offices during time not on the bench. Perhaps six month rotations instead of four.
I also like Blair's idea. As an alternative, we could assign each sitting Justice a region kind of like in real life with the appeals courts gets a Supreme Court Justice assigned to it, and if a region wants to, they can assign their judicial capacity to said Justice and forgo a regional judicial official.