Your heat and smoke rises rapidly and is bottle necking up top.
If you change the flow of heat and smoke you will be able to achieve more even grate temps.
If you were to cut a 2" diameter hole in the side of your unit, at the level of the lowest grate, rapidly rising heat and smoke would reach the top of your unit, begin to cool and fall towards the new outlet. You will need to install a stack, keep it as close as possible to the smoker body and let it extend up above the lid by about 15". You need the height to help create draft and a steady even temp. I have found it to be better to go long on the stack at first, then, trim the stack down get the perfect temp.
Check out pics of a vertical unit I built last year, size is different, theory is the same.