As far as efficiency goes, your at about 100% simply based on the use of "free" energy and very little energy expended on your part to set up and adjust. Can't get much better than that. I have not had a chance to play with it yet, but will get the cardboard and foil out one of these days and try. I do applaud your efforts though.

I agree, that as a long term survival instrument, a solar cooker is worth it as it can be used (within reason) without much tending freeing up resources (human and natural) for other things.

Bill