I recommend using airtight container for both aging process and storing in a refrigarator.
As for the dryness of soy beans, make sure your beans are very hot and moist before adding starer culture. Make sure that there is a fair amount of moisture around the beans during the fermentation process.
I’ve switched from glass dish to bamboo steamer that you find in chinese dim sum cooking mainly because I found that if you stack up soy beans too deep, the result is not favorable. Since bamboo steamer has top and bottom open to the air, I can put natto as high as 5 cm thick and they come out fine. That said, bamboo steamer is high maintenance tool as bamboo can absorb natto-kin and if you let the natto-kin stay in the steamer, they turn into molicious one which can kill good one very quickly.
If your beans come out very soft and shiny brown with slightly sweat aroma, that’s when you are done cooking.
I at least cook (pressure cooker) for 30 min and extend as it needs more cooking.
I usually buy soybeans from local coop.
If you are looking for extra small beans designed for Natto, you may need to import from Japan.
I buy soybeans from the below producer.
You may want to call them to find out their local distributor in your area.
I hope this helps.
Here are the various Q & A threads from the previous site.
My apology for not getting every threads back in the site.
I finally decided to migrate from Drupal to WordPress for the ease of use and anti spam functionality.