Well this guy has built a lovely, simple death ray and even shows his results. I like his warning page:
The sun is bright. Don’t look at the sun or you will damage your eyes. Anything that focuses the sun will only make it more dangerous. The Solar Death Ray is dangerous. Don’t build one. I’m surprised I haven’t burnt or blinded myself yet. The fumes from molten plastic can’t be good either. Don't play with fire.
From the dentally traumatized Varifrank.