start of with a box of plywood 90cm x 40 cm x 40 cm

drill a hole about 9cm diameter to allow for the extractor fan

line the interior with aluminum tape

at this stage i also installed a wire metallic mesh under which the lamps will be sitting (2 lamps 100 W each) . The metal mesh will absorb the heat and help dissipate it. It should be about 5cm height.

This is a view from the inside, all lined up with aluminum tape

racks installed along with wiring. I installed a DPDT relay which turns the fan ON when the lights go out after the temperature is reached. You can do without it and have the fan run all the time but i just wanted to experiment. 

Thermostat installed on top; You can also installed an analog metallic thermostat as show in next diagram, it's simpler as it has a dial knob and not complicated like the one used.

i prefer this one as it's simpler and easier to use: here is the wiring diagram:
simple wiring diagram
This is the food temperature chart
Dried tomatoes are just an example of the unlimited uses of the dehydrator