Does natto really need to be fermented at body temperature? I want to make natto, and recipes say that it should be fermented at ~38°C/100°F. My oven doesn't have a bread proving or defrost setting, and previous experiments with just the oven light on haven't reached that temperature. I keep my sourdough starter and kombucha at room temperature (~21°C/67°F) and it works well / I like the result. Will that work for natto?
From my research, the problem with fermenting nattō at lower temperatures is that it doesn’t inhibit the growth of other bacteria which can either inhibit the growth of the target bacteria, bactilis subtilis, or potentially introduce and incubate pathogenic bacteria which can harm your health. Natto bacteria are very comfortable with high temperatures and you should aim for between 38 and 42 degrees for a full 20 hours. 

Try looking at Natto Dad - he has a blog and YouTube videos which were very helpful to me. My second batch of nattō is currently fermenting.