Thanks for the info! Basalt structures like the Giant's Causeway were certainly part of what inspired me to do this, although I suspect that to make it look faithfully like a basalt structure would involve less greenery than I plan to put into this terrarium (I have like 10 different plants I'm waiting to restock at Glass Box Tropicals). That is a really cool idea, though! I'm going to pocket that away and probably use it for a smaller viv.
I took a pic of the structure as it stands right now, about 50% complete. This is what it looks like:
It's somewhat misleading, because there are going to be tall structures in the back that haven't been made yet, but it's the basic "isometric building" idea.
EDIT: Also, what should I use to cover the egg crate itself? I was thinking mosquito netting or hygrolon, although I'm not very familiar with the latter.