I like the idea of two burners, although I think it is real easy to get it TO HOT.
As far as the inside walls, I wipe the bottom out every time, but the smoke reside and whatnot is ok on the inside as long as it doesnt get moldy and nasty. I think plywood would just absorb and hide some of the bad moldy bacteria growing stuff. I feel like each batch gets better as the inside bacomes seasoned.
I use a real slow, warm smoke as opposed to a quick hot smike that would melt the interior.
I know commercial smokehouses use all stainless and have to bleach it out after each batch.
I also know (or think) my fish tastes better.
There is definately more than one way to skin a cat.....