Depends, does your deck boards run same direction as the pool side its on, at 90* angle?
If running same, I would look at screwing on a triangular block every 24" along the front edge with a screw into the front header in the bottom and then again from the top. Make these from a 2"x12'. Then that would allow you to lay a 2"x12" on top of them, thereby enclosing the gap.