I think when the suggestion is made to put foil around the bottoms of the plants it is meaning to put more of like a collar around the base of the plant. Kinda like the copper ones you can buy. I'm not sure if foil would have the same effect as the copper but then again I don't know what it is about the copper that deters slugs and other bugs either so.
So basically, a collar made of a circle about 6" or 7" should give you what you're looking for. And the roots extend out quite a bit further than that so between roots extending beyond the collar and the amount of water that wicks under the collar you should be ok.
Hope that helps somewhat.