When you say you poured a concrete foundation are you talking about a footer or is your entire area concreted inside like a floor?Landscape blocks from Home Depot, stacked and glued with liquid nails. You can make it any size you want, I think mine is ~5' diameter. Then lined with fire brick using mortar, and spray painted with BBQ paint. I dug out a ring and poured in a concrete foundation, prior to building the pit. I intended to cap it with some nice natural stone, 5 years later, I gave up on that idea. Lining with fireplace brick is key, that landscape block with spall!
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I would like to see some more photos if anyone has some, please.