I can't find the link right now but from memory, I basically made three mods. First, from Home Depot, I purchased a piece of sheet metal. It was probably 18x18 or so. I cut a piece from it and rolled it into a cyclinder and inserted it into the lower portion of the chimney so that it effectively extended the chimney to the grill level.
Next, I used the remainder of the sheet make a baffle extending from the fire box opening into the cooking chamber. I trimmed the corners and actually bolted it to the fire box opening by removing one of the nuts holding the firebox and cooking chamber together. Then placed the upper portion of the baffle over the screw and replace the nut. When in place, the baffle extends at about 45 degrees from the fire box wall down into the cooking chamber and is about 7-8 inches long.
The last change I made was to also purchase three sheet metal pieces. I also got these at Home Depot. They are about 6 inches wide and about 18 inches long. As I recall, I had to cut about two inches off the ends to make them work. The first one of these, I placed in the cooking chamber about two inches from the fire box and over the baffle. It rests nicely across the cooking chamber about 3-4 inches under the cooking grids and can be easily removed for cleaning. The next plate was trimmed just like the first and also placed into the cooking chamber next to the first but leave about one inch gap for smoke to get through. The third plate was then placed in the cooking chamber next to the second but leave about 2 inches spacing.
By doing these relatively small mods, I was able to reduce the difference in cooking temp in the chamber from about 70* from end to end to about 10*. It made all the difference in the world. And by the way, there is one more change you will need to make if you do some long smokes. Like a brisket or butt. You'll need to raise the charcoal/wood grid by a couple inches and/or plan to occasionally clean out the ash build up under the fire grid. (When it builds up it tends to smother the fire.)
I wish I could remember the source of this information so that I could give proper credit. I know that I got the info from someone here on the forum and like I said, it made a tremendous difference in the cooker. I now really enjoy cooking on it. Many thanks to whoever you are and my apologies for not being able to remember.