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An interesting read regarding the topic being discussed. And, it is very applicable, because almost everyone has access to an oven, or a weed burner that they could turn into a metal heater with just a few fire bricks. A very effective way of adding the properties you want to something you are building, in this case, to keep the OP's firebox from warping.And to the OP: Keep in mind that in your final product, you will want a way for the ash to fall away, so don't make the holes too small. And that it may take more than one to get to one that performs satisfactorily.
Steve