Hmm, my suggestion is to play around with it some, just for the experience of designing an Iwagumi. Believe me, when you add soil the impression completely changes, so I'll wait to give tips until you have some soil in the box. You can read my Iwagumi article in the mean time if you like, the principles laid out there will help you.
Here's a link to
one version, on APC. And a more updated version on TPT
here.