I tinkered with the plastic bag/water lens for about an hour once. Never even bothered with tinder, since it was pretty clear I wasn't getting a very good focus. I didn't think of pinning it to a sheet of clear plastic, that may have helped. I can also see this might be a little easier in southern latitudes around noon; my so-called "lens" sagged down because it had to be at an angle at my northern latitude, throwing the shape way off.