Thanks for information again, everyone. I don't know if my snake (pretty sure it's going to be my red-tail green rat snake) will necessarily be drinking the water that touches the island in the water layer; I'm not even sure if these will be true islands with mostly water underneath them or will have 90% sand (or some non-harmful substrate). But if there is a reasonable possibility that styrene will leach into water or elsewhere, I'll at least do some research on it. (EDIT: Just re-read and saw that you mentioned eggcrate in conjunction with styrofoam in order to interact with the water layer. My mistake.)
I wouldn't begrudge anyone for an infinitesimal chance of danger like that even if it did come true --- I am related to many veterinarians and even worked for two of them, and between them I could get some
idea of what killed my pet, or if not from them, from an exotics vet they know. So I wouldn't spend sleepless nights not knowing.
I suppose I can get marine epoxy from a West Marine store and paint epoxy from a Sherwin-Williams?
And if I may ask, how do people make colors / textures like the terraria / paludaria in these photographs? Not that all of them are "island spire"-like textures, but they will all be helpful to me eventually (EDIT: changed most of them to attachments below):
Thanks again for your consideration.
EDIT: Last question for now: Can I just cover the entire eggcrate structure with spray foam and work it like a foam structure from there? It may be less work at this point and besides, I have a store of like 10-12 GS Pond & Waterfall canisters.