Hi there and welcome!Seems all of the cheaper ones that I can find are constructed from galvanized steel
You are getting great info from the guys.
First off, yeah the MES30 doesn't like to have the tray inside. I've helped a number of guys with this issue involving the MES30 and the solution was to go to a mailbox mod and they never looked back.
Second, the mailboxes you find will likely be galvanized steel. Pellets burning will get no where close to being so hot that it causes gasses from the galvanized to be released.
HOWEVER, flame up situation on pellets could do so. SOLUTION... just do a burn out on your mailbox like people do with steel drum smokers.
Burn a fire in your mailbox so that the flame is burning against the top metal and releasing all the gasses you are concerned about. Once your box has been burned out with flame you won't ever have to worry about it again. Plus creosote will build up and cover your box internally with layer after layer so you will be good to go.
Here is my mailbox mod which is using a wall hanging mailbox and is super compact and modular, mine is much different from others so you get the idea you can go any which way that works :)
To do it all over again I think I will just cut a 3.25" hole into the side of my mailbox (slightly larger hole than 3" chip feeder hole.
Then I would just slap my mailbox up against the side of my MES so the holes lined up and screw it directly to the side of my MES hahaha.
No need for a duct or anything. You see how little distance there is between my box and the MES and I have zero issues with creosote, etc. so I know I don't need a great amount of a gap between burning pellets and my MES. If I wanted more gap I would just mount a bigger box that provides more space/gap.
I hope this gives you some ideas. Thanks! :)