Sorry you have notten gotten any replies on this.
—-can’t help with the firebox size...sorry, I just don’t know
—-2 inch insulation is pretty standard.
—-11 g would be appropriate for inside the firebox but overkill for the rest of it. Won’t hurt, just don’t need that thick if you are going to insulate it.
—-if you are going to run a water pan, the key is to make sure it will hold enough water for your entire cook. Adding cold water during a cook could cause controll issues. I would want to start a cook with about 3-4 quarts of water.
—-I would make charcoal basket as big as possible. You can always fill it half way For a short cook but you can’t over-fill a small one for a long cook.