A piece of innertube about 1 inch square.  Light one corner, put under tinder.  All done. 
So it's pouring rain and all you have to do is whip out a 1 inch piece of inner tube,light one corner and toss it under your tinder ?  I guess I need to go back to the drawing board because it's never been as simple as that for me.  I must have been doing something wrong all these years.  

Are you lighting the innertube with napalm ?