The pan in the middle would seem to divert most of the heat up and out the sides, which means it will take more to keep temps. You mite try putting some holes all around in it. A second option would be take the tray out and put in some adjustable metal strips that can be opened or closed depending how you need to regulate where the heat will rise. Try to get some insulation around the fire box, and maybe the top as well, even a welding blanket over the top will help, also you may be able to raise the level of the fire grate so you are not heating the whole barrel being used for the fire box.
Put some metal flaps on each end so the heat will deflect more to the center mite help heat rise up and around to the exhaust. With the way you have it set up putting one exhaust in the center of the lid, keeping the side exhaust closed may get a better draft movement. These are just some ideas, and I really have no idea if any or a combination of them will help. Try taking the center pan out, I do not use a diverter in my uds which works just fine like that, you have enough room to move meat around so you are not cooking direct if you don't want to.
One last mod, put some fire brick around on the inside of the fire box, but that wouild take a lot of bricks.
With this cooker that looks to be well built for what it is, you are most likely never going to get your usage down real good without some mods.
Dave