Wait...you're not aerating your koji? No wonder it's dying on you! Aspergillus oryzae is highly aerobic, man. That's the one of the reasons why you have to take the koji out every few hours and give it a stir. If you're keeping it sealed up with no access to oxygen, you're suffocating the mold to death.
I feel ya on the 12 hour shifts, man. I've worked on the Alaska north slope. If you can't be available to stir the koji once in a while, then you need to come up with a way to ventilate your incubator. How are you going to do that with a cooler? I don't really know. I only suggest using a cooler because they're cheap, portable, commonly available, well insulated, and don't need any assembly. The caveat is you have to be available to babysit your koji for 40 to 50 hours.
Edit: if you're just trying to make tane-koji to inoculate future batches of kome-koji (to make sake with), you don't need an incubator. It will take more like a week, and the result is unsuitable for brewing sake due to low enzyme production, but you really only need room temperature to successfully make tane-koji.