I'm going to be building a stainless steel insulated smoker (dual wall). My shop manufactures food production equipment so I have access to anything I'll need to complete the build. I'll be tig welding it together. I want to do a horizontal offset with a vertical attached. I want it to have dual shelves in the horizontal and about a 36x18 cook surface in the horizontal and the vertical will be about 48x20x20. I need to know how big the fire box needs to be.