Hi Leroy and welcome.
What I did on the two inner walls on the Dyno V was the first inner wall near the front of the stove is the same height as the outer walls reaching from top to bottom.
I welded that wall onto the bottom plate before welding the two outer walls.
I then welded in the second inner wall which I made one inc shorter that the first inner wall.
This allowed exhaust gasses to move to the chimney section via two paths which I explain in one of my videos.
I was able to weld the top plate to the fist outer wall from the back side of the inside of the stove right at the chimney section which I welded on last.
The back burn stop measure I had employed was a simple one, to make sure all my doors and air feeds would be air tight.
By having an airtight door on my pellet feed even if it did back burn I could easily shut the entire stove down and starve it of oxygen.
It appeared to work quite well during several testing phases and I did not have one issue with any burn back taking place.
I am not saying that it will do this 100 times out of a 100.
I never had enough time to do the kind of testing that I would have liked to do.
Now with that thought I would warn against using any gravity fed pellet stove over night unattended.
Bigalow farms scared the daylights right out of me with what happened to him.
I would never trust a gravity feed system with my life unless I had hundreds of hours of testing and some sort of automatic safety system that would shut everything down.
I just got a note from Vince33 on some exciting things he has been working on that monitors a rocket stove.
He said he will be posting his work soon and I will say this it is the future of the wood and rocket stove industry.