Thats exactly what I was thinking, the only thing I'm trying to figure out is how to coat the foam, for that I'm going to troll the dendrobate board and see what they use.
did someone say tiered pools?
expanding foam is probably the easiest to work with, but sheets of pink/blue foam will work too. there are several ways to seal the foam too. cement/grout, drylok, silicone+coco coir and epoxy all come to mind and are the most common. or, you could do what i did and just use containers that don't need to be sealed. they don't look as natural as something shaped and then made to resemble rocks, but they were easy and only took a few seconds to make. they are also really cheap.