You can incubate tempeh in a warm place in your home. A closet with a drop light, a place where heat collects, such as a high kitchen shelf or warm attic.
Plastic bag: Place two 6” x 8” Ziploc bags (quart size) on top of each other on several layers of thick clean towel. With a clean straightened safety pin, poke holes in both bags every half inch in a grid pattern. Divide the beans evenly between the two bags. Cover the cookie sheet with two layers of thick terry towel and set the bags on them flat. Pat beans firmly into an even layer making sure they fill the corners of the bags.
Cake pan: Pack beans in an even 1/2” deep layer in a very clean, oil-free 9”x13” cake pan. Cover with aluminum foil and perforate every inch in a grid pattern with a safety pin.
Incubators: A tempeh incubator should maintain a temperature of 85°F for 26-30 hours (85°-93° is best). Don’t incubate in anything small or airtight because the culture needs oxygen to grow and some “breathing space” to discharge excess heat and humidity. A couple of practice runs with only a thermometer in your incubator before making tempeh is a good idea to check for temperature fluctuations that may cause problems.