An idea I've been thinking about would use a large fresnel lens like the kind on the front of big screen televisions. You used to be able to get them from Edmund Scientific, but I think they might have stopped selling them. That lens on a bright day will achieve temperatures that can melt steel. With that placed over a pit lined with fireplace concrete you have an incinerator that can't be beat. Best of all it uses no fuel. I don't know if there are any technical glitches that would prevent this from working since I haven't actually built one of these, but other than overcast skies, I don't see any reason why it wouldn't work.
