Corey,
I foil just the part of the charcoal grate that does not have charcoal on it.
Yes, foiling the kettle bowl would be a PITA, I have never considered doing that.
I generally use heavy duty foil, just because I have it, 18" wide (I got a 500' roll of the stuff at Sams Club) I carefully place it on the C-Coal grate, and I let it overlap the edges of the grate, so I can kind of form a flimsy pan of foil, the shape of the grate and slightly rolling up the sides of the adjacent bowl areas, done carefully, very little if any run off of juices or marinades occur. After the cook, I just carefully remove the messy foil, and toss it!
This also helps direct nearly all of the incoming air directly to the area below my fire, instead of some of the air just drifting up through the open parts of the grate.
So I believe this foil arrangement also aids in getting a more consistant air flow to where I want it (my fire), and I can control my temps a little better this way.