The Double and Triple signs were indeed single frames, around 4 1/2 feet high, and they would hang on the board, rather than slide into any sort of slot.
In much the same fashion, the "Family Feud" sign that would cover one side of the board at the open of the show would hang over one of the sides of the board normally used throughout the main game. In fact, if you watch some of the shows' opens closely, you'll notice that the sign was only attached at the top, because the sign appears to want to try to fly away from the bottom when the board is rotated.
Ah, centrifugal force.