Time to confess.....
I bought this stainless proofer/warmer at a school surplus auction for around $85 quite some time ago with the intention of converting it to a smoker. Problem #1, it's too tall for where I want to put it but I just can't part with it as it will be an awesome smoker. The heating element is on top and it has 2 convection fans that suck air from the proofer chamber, blow it across a finned heating element, and then down a 2" channel on the right side of the proofer interior. This channel has louvers all the way down and it evenly distributes the heated air. Wife keeps threatening to bury me in that thing LOL!!! It's big enough.
My point is, you may want to consider something like this for your build. The proofer I found was a McCall model and it's all stainless inside and out.
This is a diagram of what I'm talking about (and what I intend to do "one day").