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Add a thin layer of mortar along the outside of the frame to add a smooth finish (Image 1). If desired, cut pieces of slate to fit the top of the frame and mortar in place (Image 2).
Add gravel about three quarters of the way full, so the top of the fire bowl will be flush with the top of the concrete frame.
Add or subtract gravel accordingly to bring the bowl to the correct height.
Add a large cobble, such as Mexican pebble, along the outsides of the bowl until only the top rim of the bowl is exposed.