I have finally gotten around to filling open air voids with water resistant rigid spray foam made by Dow (12 oz can). I ended up using three cans.
Using a drill bit the same size as the spray nozzle, I drilled a hole behind the latter area and two holes in the area of the compression post. Put a piece of tape on your drill bit so you won't drill deeper than 1/8 inch, to prevent drilling into the outer hall. Tap the floor with your knuckles to find the greatest cavity sound to drill into.