Are you planning to have upper and lower gas burners?
No one will like this answer but I'm still not decided! The extra complications in build would be offset buy the fact that everyone here can relate to the concept of 2 independently controlled burners.
There is the problem that there is more stuff that can go wrong in the future. However without some way of separately controlling upper and lower heat it seams that that getting an even cook is a bit chancy. If I have too much stored heat on the ceiling or the ceiling is too close to the deck then I will consistently have over browned tops. Conversely, the opposite could also be the case.
The main disadvantage that I see with a top burner is the requirement to make the gap between the ceiling and the hearth wider. About 2" wider should be enough. I want about 9" of work space so that would make the gap come to 11".
I would say that overall I am leaning towards incorporating a top burner. Having said that some mornings I wake up chastising myself for trying to reinvent the wheel and determioned to make do with one burner!
On the aluminium plate question. Would this be considered, what you guys call "the deflector". Is 6mm thick enough to avoid warping. Is $100 going to be an unnecessarily expense? Should I make it $200?
Thanks for your thoughts.