I had spend a lot of time looking at very complex home-made solar food driers on internet. Most of them consist of a black area which heats up the air. The hot air dry then flows over the food to extract the moisture.
I realised that a very quick free version can be achieved just using a back tube and a metal colander. The air heats up in the tube and then passes through the holes in the colander.
This drier can dry out a couple of apples in about 6 hours. It is a bit unstable and can easilly be knocked over. A good variation would be to use several black tubes taped together with a big cardboard box on top.