Does natto really need to be fermented at body temperature?
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.