For mine I did a few things that made a world of difference. First off, make a new fire box. The way those grills are made, your fire sits right in the ash pan. On a long cook, the ash will choke your fire and it will progressively get cooler, demand more fuel. I put a few long SS bolts on both sides and made a grate from expanded steel with a square steel tube frame. It just rests on the bolts. Now the fire breathes great, and I can even pull the ash catcher and empty it with a fire going.
Second is a baffle. I cut a 12" x 24" piece of 16 ga steel to fit, and fabricated 2 hooks so it could be removed if I want to grill with direct heat. I didn't bother with tuning plates... I just use one of those giant turkey roaster foil pans full of hot water, about 6" from the baffle. I'm getting temps +/- 10 degrees from side to side.
After that, look into getting an oven gasket. They sell them at Home Depot or you can get them online. Try to seal up all your leaks, which with a chargriller are many.
You can also extend your chimney down to grate level with some dryer duct, but I never bothered with that one... Really the main things to go for are the baffle, fire box, and seal.