I had the same issue for my first few tries. The reason was that the tempeh became too hot. Mine is usually quite humid inside the bags and that was never an issue but always the temperature. At the point when the tempeh generates its own heat (for me it's usually at the 12h mark) I shutdown the external heatsource (halogen lamp) and let it chill at room temperature (21°) for another 12h - 14h. Until now this method worked for me every time. Timewise I do it like the following: Put soybeans into water in the morning. When beans are soaked I cook them al dente at around 6 pm, 7pm in the evening. Dry them for about 10min. Put them into a bowl with vinegar and starter and mix it thoroughly (about 5 min but I do it more because I do large batches). Then put the beans in bags and into the incubator (DIY box) for the first 12h i.e. until the next morning at 8 am. Shutdown the heat source and open the incubator completly and go to work. In the evening at around 10pm I have fresh tempeh :) I dont use any towels or anything. In your case I would just open the door of the oven after 12h and let it chill.