Meat, It looks like you may have too large of a fire box to cook chamber opening. But it could be just the pic. Im building one simular to yours, only a triple, and was surprised on the calcs from fire box to cook chamber size requirements. Have you tried to block part of it and see what temps do?(edit) I see in the pic now that you welded part of it up(edit) Maybe add to the other side.
Also maybe try a bigger (if possible) heat diffuser and or toss in some water pans on it to catch grease from stokin the fire up.
Some sort of a damper would be worth a shot if its not to much of a pita to get in there.
Keep tinkerin, you'll get it.
UDS, XL BGE, and a pane burner.