Here's a brace of solar cookers I've just made, while in the waiting room. The small one (left) is making hot water for a shower or washing up later on today. This simply has a 20L solar shower bag in it. The other one (my own UK design adaption) is currently cooking the famous baked bean recipe mentioned on the forum, and is actually boiling as we speak.
They work in the UK fine, even in the winter, but they do need good clear days of uninterrupted sun when out of season which puts the spanner in the works in the UK a little. I have boiled water on the 22nd of December in the UK, so I know they can work in theory. I made some focaccia the other day too, which was very effective, and only took an hour or so to cook after 4:00pm. They do not burn food, so there's little danger of overcooking, so cakes are very good, as are stews etc.
They fold flat when not in use, and are based on the "All seasons solar cooker" from the internet which you can Google if you are interested.
You can make one from cardboard boxes, kitchen foil and PVA glue. These ones are made from Estate Agent signs and aluminium repair tape, so are waterproof.
They would definitely work well in Southern Europe.
I need to try them out when out and about in the Cali, which still hasn't arrived yet, but watch this space....
If the results are consistent, I'll post some more about the project.