I use multiple pieces of foil. With skinny foil 3 and with the wide foil 2.
I have the older 18.5" WSM and the Reynolds Heavy Duty is just wide enough to cover the bottom using one sheet. I understand the new 18.5 WSM uses a larger water bowl, so it may not work for that...
I've got a question. why are people foiling the bottom of the water pan? I've seen several posts about it here and just curious, I just got done with my first couple of cooks and did foil the top for easy clean up and man did that work out great! thanks justin