I like Thels idea from above, I think I will attach it to the Highlands board, when the next corner slot is taken. But yeah the initial idea was to use my 2nd edition board as the Dragon region. Any board will do, even the board JediKnightAmoeba suggested.
Its totally upto you but making a region that can only be accessed to (baring teleporting cards) a already made region expansion. Maybe could you make a sort of inverted "T" board to fit between two side boards accessed through the middle of the longer part of the outer region..