re: Solar Kiln for drying lumber

Nicely done.  I would like to make a scaled down version for my driveway.   Did you put a moisture barrier underneath the concrete slab?  Concrete can wick up a lot of moisture from the soil so you may need to keep an eye on that.  That may be partly why they recommended a wood floor?   It might be beneficial to put plastic over the slab to help deal with that.  If it is an issue, you may find that  there is condensation on the underside of the plastic and it could vary by season and with changes in weather.   Just thinking out loud.