I wouldn't recommend fermenting food yourself.

Instead, you can preserve natto by freezing them.

Defrosted natto are still as good as fresh natto. I always bought frozen natto when I was outside Japan and never unsatisfied me.

So, if you like natto, I would suggest:

- Buy plenty amount of natto when you have chance, e.g when you come to Japanese grocery store far from home

- Preserve them in freezer

- Find a best timing to defrost natto using your microwave when you eat!

That way you will not fail eating good plate with natto!

This might not answer your question but hope helps in differently for your happy nattoing!