I dug my baking sheet foil out of my BOB, folded a 9" X 8" piece into a single layer cup, and set it over a can of Sterno. There was one pinhole at the very edge of the foil that I easily avoided. I had bubbles within 7 minutes and a full boil within 10. It did not come to what I would call a rolling boil like you would get in a pan, but I don't think that is very important. As long as the water reaches boiling temps you will be fine.

One interesting thing I noticed was that the foil cup boiled much faster over the Sterno than my Campmor steel cup does. (10 minutes vs. 18 minutes.) I attribute this to the uneven surface of the foil cup acting more like a chimney (for want of a better word) than the smooth surface of the cup, or maybe it has to do with the properties of aluminum vs. steel.