Gee Ned, I don't know... I know the new water pan is larger than the old one, and the 18" foil just makes the turn over the top of my older pan. I don't think using 2 pieces to cover the bottom will be a problem.
As far as the inside goes, don't push the foil all the way down to the bottom on the inside - you can leave an air gap there. A lot of people will ball up some foil, put it in the bottom of the pan, then foil across the top pushing the foil down just far enough to touch the balled-up foil. I just let the foil sag a bit over the top - just enough to catch any drips and not let it run over the sides.
Another option is the foiled clay saucer inside the water pan... there are a lot of posts about the saucer on here.
Hope this helps....