Creation Ė got your PM. A few quick comments.
I love your workmanship, the upper IR burner (esp. adjustable legs), the overall look and balance of the setup. Really good stuff here.
Itís hard for me to visualize the heat flow.
Looking at the backside picture, Iím guessing the lower, stainless-colored vents, are the air intake for the lower burner(s) and the upper, black-colored vents, are the hot air exhaust. Right? Did the original hood for this grill have an air vent in the upper area above the burners? It must have had an upper heat vent (like mine) in the rear of the hood. Otherwise the heat from the lower burners would only exit the back of grill never going above the grates when the lid was closed Ė I donít think they are designed like that.
In your modification, assuming the top hood just fits over the grill, it makes sense that some of the heat from the lower burners are blocked by a square pizza stone (? - I donít see any evidence of a rotisserie or round stone) and exit the black area in the back. The rest of the lower heat flows up the sides and into the hood building up until it exits the forward cutout slot. Given the likelihood of hot air buildup in the upper hood, that top surface must be ďbugger hotĒ unless the stainless is double walled and/or insulated. (Correct me if this is not true b/c it means I donít understand the hot air flow in this system.)
That said, what youíve created is beautiful, but it doesnít work right. IMO, take off the hood and prototype something else with a roll of aluminum flashing from Home Depot and rivets. Iíd want to understand and control that underside hot air flow. Iíd block the side grates, maybe a little blocking in the front, so everything is forced out the back in the black-painted vents. In my case I forced everything to the back of the grill then added a diverter to channel the hot air flow over the pizza and out the front slot. (A lot depends on the stone Ė square or round Ė as to how you want to do this.)
Now that Iíve got the lower heat flow controlled, Iíd think hard about letting the IR burner do what it does best: radiate. For this to work, I think you have to use a rotisserie, otherwise youíll get a rectangular char on the round pizza mirroring the shape of the IR burner. I think you understand what Iím saying even though the char wonít really be exactly a mirror imageÖ
You might look at pictures of salamander broilers and/or cheesemelters. They have gas versions that use the upper IR element and therefore manage the air intake and exhaust. The general rule for grills and broilers is take the cool air in from the front and exit the back. Only by hacking together prototypes will you get a sense of what works.
Regarding the vent/chimney idea, I think it has potential. A flue doesnít do much IMO. All you need is the vent, but angled away from the chef. Again, Iíd prototype first. Iíd try the vent at the back of the IR burner. Assuming you donít have a lot of heat flow from the bottom, venting behind the IR burner should draw fresh air from the front slot. But if you prototype it right, maybe you can switch the vent from front to back, or even to one side or the other of the top surface of the hood.
Iím out of time right now. Iíll keep thinking about it. Answering some of my above questions would help. BTW, where are you located?