I'm building an evap for a 2x4 divided pan using an old fuel oil tank. It appears that most builds have a "smokebox" at the back of the unit that the stack sits atop (the the mailbox-shaped part that you don't cut off the top of the tank when making the pan table). Is there a thermodynamic benefit to this? Seems it would be easier to cut the top flush off and have a flat area behind the pan that the stack is mounted on - then there's a bit of flat surface that can be used as a warming table.

Thoughts on this are appreciated!

I'm sure you all know what I'm talking about, but just in case the first pic is with smokebox, second without: