I think 9" for the base is pretty small. I used 12" and I think Danny likes to use 14" For filling the inside, you could cut some wood squares that will fit the opening, cut a hole in the center of each so a 3" pc. of plastic pipe (DWV) will fit through. Glue the squares (2-3) into the opening, and insert the plastic pipe capped on one end. Fill pipe with sand, and cap the top end. This will give you 2-3 internal braces, and a good sized stack of sand.