You need to dig the base out 12" deep pack the bottom with sand to 4" lay down some wire mesh( old cattle panels work good ) fill with concrete to a deph of 6", let it set for a week. Lay down fireplace mortar and top the slab with fire brick then start up the walls two bricks wide. frame the openings with 3" angle iron an put "T" channel in around the perimeter at grate height, go up 2 more rows and add a top cap. The lid can be made from 11ga. with a counter weight system.
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