Muidaq, a couple things.
First, the jury is still out on a foil ceiling (Kenji is for it, I'm against), but, with the temps of the charcoal, aluminum has no place anywhere near the flame. No foil on the charcoal grate and definitely no aluminum pan as a deflector.
A deflector for a Kettlepizza setup is definitely not a bad idea- if you can get steel. Prior to the most recent upgrade
to the KP, both Kenji and Elijah seemed to be getting good results solely with fire placement- pushing the stone hearth as far as possible to the front and pushing the fire (with some wood) to the back.http://slice.seriouseats.com/images/20120530-pizza-kettle-test-three-works-01.jpg
You definitely need a ceiling that covers the entire area of the upper grate. Foil works very well, but, like I said, a strong wood fire could push the ceiling to 1100+, and, if it did, the foil would melt. It's probably not going to give you a problem, but I'd still be aware of it as you burn bigger and bigger fires in an effort to reduce your bake time.
As Kenji found out, the ceiling is less about storing heat and more about trapping hot air (although both are advantageous). Whatever ceiling you go with, it should be as air tight as possible to force all the hot air through the door.