The plank should be 3/4" to 1" thick, just enough to be skratshed from above through an end-lid, long as the plank broadwise.
It will then have a surface of ~2/3 of what the silo cylinder has.
That is an effective heating surface compared with the Imbert mantelling, as that one looses 60 ~ 70% of the heat to the crows in the cold streaming wind.
In the lower end the plank divides into two legs allowing easy flow of wood into the “mouth” of the firetube.
So these legs end into the cone connecting silo bottom-end and the mouth of the firetube.
Outside the firetube we can have an (traditional) air preheating cylinder…
A bit at a time…