Morrey,
First, I don't use water. I use a foiled clay saucer (a planter base).
On the 18" it was never a problem for me using 18" foil. I would put a couple balls of foil in the bottom of the pan, and the put foil over, and it was still catch on the sides.
Since getting my 22", I have always ended up with massive amounts of grease in pan. A lot of times we were cooking 80-120 lbs of pork on there, so I just thought that amount of grease would always make it's way through. Well a few cooks ago I tried something different on the 22.
First, I foiled the bottom of the water pan using 2 sheets of 18" HD foil.
Next, I used 1 sheet of foil on the bottom of the clay saucer. I then foiled the top of the clay saucer.
Next, I took 2 sheets of 18" HD foil and sealed them together. I started with about 4" of overlap and folded it 1" at a time. Then once it was down to 1" of overlap, I folded it in half.
Now here is where the leak issue comes from. As soon as you put the weight of the grease on this seam, it will spread and liquid will find it's way through. So to combat this I lay the sealed together foil on top of my clay saucer. I push it down to follow the contour of the saucer, making sure to keep the seam tight. I then dropped, gently, the saucer into the water pan and sealed the foil along the edges. This way when grease falls into it, the weight is supported by the saucer.
I have done 3 cooks this way since, and so far only ended up with a couple drops of grease on the foil on the saucer. If I only have to replace that 2 pieces of foil each time, and then the top one of the saucer every few cooks, I will be happy.
Also, they make 24" HD foil, but it's $90. I will stick with what I get at restaurant depot, it's $22 for 500 ft of 18" HD.