Fresh rice koji, just after completion. This batch is going to go into white miso.
The vessel in the photo is my current favorite for koji growing, it is a repurposed rice tub for sushi. 5 go size, about 38 cm (15 inches) in diameter. 1.5 kg (a little more than 3 lbs) is the easiest amount of rice to grow in this tub. 2 kg is pushing it. It tends to overheat if I put in too much.
I have used pretty much anything and everything from plastic trays to cast iron pots to stainless bowls as koji growing containers. I have found that as long as I know what to aim for, I can grow koji in it. But this tub works the best for me. It is true that wooden vessels help control the moisture and prevent both condensation and dryness.
I have a growing appreciation for tradition since I have started making miso. All the little traditional ways have meaning. Someone in the past have made all the mistakes and have done all the experiments. I tend to still want to verify by myself, being the skeptic I am, but learning from the past is the quickest way to success.