My Swiss chums use one all the time when they're here in the summer. If ever you remember what a telex machine silencing cupboard looked like (and why should you?) their's is the same - a cabinet lined with tin foil with a glass or acrylic lid.  Put your casserole or jacket potatoes or what have you in there first thing in the morning, orientate it to the sun and eat for free at lunchtime.  Think about dishes that benefit from long slow cool cooking.  Rice pudding?  soups? meat that's been browned off first on a cooker? Maybe a chicken so long as you finish it off at a higher heat?  Endless choices.  Free, fab!