Franco, good to see the evolution of the idea. I particularly liked the idea of using the bottom of a drinks can. Weighs next to nothing and would also collect any melted wax that spilt over the edge of the Tea Light its self, making it easier to wipe the cup out after use. I could also use the bottom of the drinks can as a small priming pan for my alcohol stove if necessary. I am looking for smaller plastic cups which will make them easier to store but retain their use as cups rather than permanently attached to each other. However, I see the appeal of a dedicated unit.
Bob. Whilst there can often be room for improvement, I can't see how two cups, one stacked in side another is over engineered. You back your statement up with your observation that the cups were still cool after an hour. Surely that is the point, to keep the cups cool.
I cant see how using olive oil would add anything. Water is more readily available and if it spills it will evaporate without staining items etc. Oil can be messy and I would not want to be having to empty the lantern of oil after use. Oil can also be flammable if the right combustion temperature is reached and so adds an unnecessary complication. Given the small amount of water held between the two cups I can't see how suggesting it be used to warm up water is of any practical use let alone using the lantern with oil in to cook with. I don't know how well dental floss would work as I have never used it for any application other than flossing.