For all those enjoying to draw new borders, here another case: Schleswig, the northern part of which was returned to Denmark after WW I. To enhance the fun, you get several maps:
That's easy, you just place the border at the Eider.
Joking aside OTL border while not perfect, was not a bad choice, yes I personal think that Denmark should have adopted an Eider border after WWII (without expelling the Germans and it should respect their right to their own language) where we could have gotten away with it. But with hindsight it was no disaster that it didn't happen.