How to build your own incubator
There are many expensive incubators on the market but it is very easy to make your own tempeh incubator. All you need is an old fridge (which you can get for free at appliance shop, they will be happy to get rid of it) in which you install a lamp. The heat produced by the lamp should be controlled with a dimmer. An old fridge is the ideal incubator: it's already isolated, easy to clean and it contains racks. Once you have made your incubator you can also use it for other purposes, for example to make yoghurt or to let your bread dough rise. It only cost you 10 to 20$ to make your own incubator. This is all you need: an old fridge or other box, a light dimmer, a light fixture, electric cord with plug and a 25W or 40W light bulb. The lamp fixture should be fixed on the bottom at one side of the fridge for uniform heating. Most fridge have a hole at the back (to let the condensate escape) through which you can pull the electric cord, otherwise you will have to drill a hole.
Controlling the temperature
It's important to check the temperature during incubation because the tempeh , especially at the end of the process, will produce its own heat. Also ambient temperature can change.
The temperature of the incubator should be controlled between 28-33°C, or ideally between 30-32°C. Therefore you have to adjust the dimmer dial. Remember that the most import cause for spoiled tempeh is too high temperature.
If you want to invest a bit more you can install a thermostat (eg a chicken incubator thermostat) and a ventilator to control the temperature.
Use a cardboard of about 30 x 30 x 40 cm or larger. Isolate the outside of the box with polystyrene.
Put the carton in the upright position and place a light fixture + 25 W lamp at the bottom, connected to a light dimmer. Put a rack at 2/3 from the bottom, attach it with nails which you put trough the carton.
Modern electric ovens with a ventilator can maintain a temperature at around 30°C (check with thermometer).
Of course you don't need an incubator if you live in a tropical country where ambient temperature is ideal for growing tempeh. Remember that tempeh originated from Indonesia with a warm climate the whole year round.