Hi Peter,
Here are photos of a design ECHO East Africa promotes shown to us by Anja Weber. This design uses only one piece of plywood, has variable airflow through an adjustable vent, uses expanded metal over tin foil in the flat-black painted convection heat chamber to maximize heat, and costs just $100 including construction here in Tanzania.
Here are the plans for the dryer: Solar Dryer Plans.pdf (6.9 MB)