Ideal humidity is probably 90-100%. If you want more humidity, you can put some soaked sponges or a tray of perlite soaked with water in there.
But I have a feeling the problem was the temp and/or too much competing bacteria. Next time you might take a little extra care to make sure your prep area, utensils, blankets/towels/whatever, are extra clean. Avoid using wooden/porous utensils that were used for other ferments. Bacteria grows much faster than fungi, and if you give them the advantage, they'll take over.
You may need to monitor the temp a little closer...it builds up over time. Cracking the lid on the cooler is one way to reduce temp. Other options get more technical/expensive. You might also try doing a "dry" run with your hot water bottles in the cooler (no koji) and just see where you're at. IIRC the ambient temp in the cooler starting out should be between 80-85F.