Center your concrete ring atop the pit. Next, build a second form in which a slab of concrete four feet square and 2 1/2" to 3" thick may be poured. If this slab is poured flat, a wooden riser box with cover will have to be constructed and mounted on the slab later. The riser may also be made of concrete and poured as part of the stab. All concrete should be suitably reinforced with steel rod, with eye and anchor bolts embedded in the sill and slab as needed.
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