Xisca,
(I think I just saw your name in the forum on my comment for Sandor

Is your plan to build an updraft solar dryer or a down-draft solar dryer? I'm intrigued by your idea to use stone, but it might prove to be a difficult material to work with as far as a conventional dryer. My dryers have tight seals except for the vents/chimney and the air inlet, so I'm thinking about how many gaps you'll have in a stone wall (I'm inexperienced with this idea of stone drying, so that may not be a problem)
Also, which way am I facing as I look into your picture. You said that it was going to face south. Is south to the left in the picture? I have some ideas coming to mind right away that I'd love to sketch out for you and scan in as a picture so you can see, but it will help if I can orient myself to that picture. What sort of materials do you have to work with for drawers, solar heat accumulator, and chimney? If you were to build an updraft dryer I think you would just need wood/screen/lathe for your drawers, and the rest could be stone. If you were going to do a downdraft, it would take up less space if you used wood for the internals to separate your down draft cavity and the chimney on the back.
I really hope this works for you, and I'd love to help in any way I can. I've progressed from simply drying on screens in the open sun, to updraft with/without fans, and now I'm building a solar downdraft dryer out of salvaged wood and stovepipe. I'm definitely no expert, but I've had fair success drying food and I'd love to be useful to you. Cheers
Adam