I've posed the same question on the Next Scotland Manager thread and suggested a job share style arrangement between club managers.
Failing that, I think an 'inside' lower profile appointment like Gemmill or Mackay should be handed a long-term contract to manage both the full squad and the Scotland Under-21s. There seems little reason that one manager can't do both and it would also provide a more hands-on experience instead of months of inactivity between games as well as giving the manager a better overview of the players at his disposal. The constant changing of manager doesn't work at Scotland level. If it did we'd have qualified for something since 1998. Bar Levein who thus far has won nothing at any level, all those in charge since then have been relatively successful club bosses but simply haven't been able to translate that approach to international level. An appointment like Gemmill who, like Gareth Southgate, has worked within the international set-up for a long time would make more sense.