Have always thought you needed it off, just from reading I am no expert. Even at the end of my boil I doing cover it when cooling until vapor quits rising.
I would think you could take a chunk of cloth, in the center tie a string, then hang it over the pot so it looked like a "dunce" cap; to retain heat.
The vapor would soak into the cloth, when it got dense enough it would dribble down the cloth and away from the beer.
The only think I can see as being an issue is where does the water go,,, we don't want it to put the burner out.
Splurge and by yourself a "real" burner. I have two of these though I got them cheaper than this http://www.campchef.com/maximum-output-single-cooker.html
It works fine for ten (10) Gallon batches and will get 13 Gallons wort way past a modest "rolling boil"