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I wouldn't go less than 1/4 in ...... preferably 3/8 on a firebox if I were doing a build. The thicker steel offers better heat retention and won't rust out anywhere near as fast.
Sounds like DougE
has you covered. The firebox is always the first thing to rust, so it’s always better to have extra thickness if you can to minimize warp, hold and even out heat, and last longer.