Use the upper grate for the meat. U can place the water pan on top of the bottom grate. This will provide better access to the coal pan. Also i forgot to mention to put something underneath the cooker to catch the sweat and prevent catching your deck or grass on fire. If you use do modify and put vent holes in the side of the unit make sure you have a good means of closing them off. A couple of thick sheet magnets will work well. you might also consider drilling a smoke release hole in the lid to help further regulate temp and smoke.
Best of luck and let us know how things turn out.