if it was above the firebox, you wouldnt expend any energy to heat it up, the radient heat would heat it up anyways, but, if you will be able to shut off the vert box your design would probably work great, but, i think it would have to be a "true" reverseflow, not with tuning plates one solid plate over to the left dumping into both the vert box and wrapping around to the top.
now, would you use a simple "cookiesheet" type deal just a flat piece of metal slid in place to block off the vert chamber? the stack on the other side could simply be a flapper that closes sealed
Til next time, chew carefully, and dont let your meat loaf!
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