By the numbers, you are pretty close for your firebox size compared to the cook chamber.
I was going to try and give you an idea for how much offset you needed for the firebox but it seems that I have a bad link to my circle calc that I use.
What I did on my RF round tanks, was find the area of the football shaped piece between the two tanks and make the dim match the area needed between the fire and the food.
There is no problems if you make the fire to food opening a little bit bigger then actually needed and in some cases it can even help out if you have it set up as a damper.
Personally I don't care to get the face full of smoke every time I have to get in the rig. So I like to have a damper that I can close off if I want to. One or two things I dont care, but when I have to move a ton of stuff I hate the smoke in the face.
Its also to have the RF plate gap a little large and run an adjustable damper there as well.
For your question about the air intake.
If it said 9", then you would want one 9" hole cut clear with a cover that could slide over that opening.
For a 9" opening, I would use two 10" butterfly dampers.
You can use any number or shape of openings so long as you give it the recommended area for air intake.
Here is a less then perfect vent on one of my smokers. This vent is about 7 or 8 inchs and the rig is 42" for the food and 22 I think it was for the fire.