When I tried the "polish the bottom of a soda can" thing, I had lousy results until I smeared some charcoal onto a piece of white cotton (gun cleaning pad, actually). I think the main thing the charcoal did was to increase the amount of light absorbed by the cotton. I got a "coal" this way I was able to start a fire with. Wrapped the smouldering pad in a handful of dried grass and blew. Worked better than I ever expected. <img src="/images/graemlins/smile.gif" alt="" />

So. I'm thinking it's all about the tinder. Maybe try the standard cotton ball / vaseline but with something on the cotton to darken it up enough to catch. I'd imagine the fresnel lens would work at least as well as (and probably better than) the bottom of a coke can.